UC-NRLF 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

Class 


STANDARD 
POLYPHASE  APPARATUS 

AND 

SYSTEMS 


BY 

MAURICE  A.  OUDIN,  M.  S 

%  i 

Mem.  Am.  Ins.  E.  E. 


WITH  207  PHOTO-REPRODUCTIONS 
DIAGRAMS,  AND  21  TABLES 


Fifth  Edition, 
Revised  and  Enlarged 

NEW  YORK: 

D.    VAN    NOSTRAND    COMPANY 
23  MURRAY  AND  27  WARREN  STREETS 


LONDON: 

SAMPSON    LOW,    MARSTON    &    COMPANY 

LIMITED 

100    SOUTHWARK    ST.,  S.E. 
1907 


COPYRIGHT,  1899,  1902 

BY 
P.  VAN  NOSTRANP  COMPANY 


Stanbope  press 

Pi      H.      CMLSON       COMPANY 

BOSTON.        U.    S.    A 


PREFACE   TO    FIRST   EDITION. 


THE  development  of  polyphase  apparatus  and  the  appli- 
cation of  polyphase  systems  to  the  solution  of  engineering 
problems,  have  been  so  rapid  and  varied  of  late,  that  there 
is  no  available  literature  on  the  subject  which  is  at  once 
practical  and  up-to-date.  The  excuse  for  this  little  book 
is  the  demand  for  information,  in  a  convenient  form,  on  the 
characteristics  and  uses  of  the  various  types  of  polyphase 
apparatus,  and  on  the  actual  working  of  the  several  poly- 
phase systems  now  sanctioned  by  the  best  practice. 

These  notes  are  intended  for  electrical  engineers,  cen- 
tral station  men,  and  others  who  talk  about,  operate,  or  are 
interested  in  polyphase  machinery..  While  a  certain  gene- 
ral acquaintance  with  alternating-current  apparatus  is  pre- 
supposed on  the  part  of  the  reader,  the  author  believes  that 
the  reader  whose  experience  has  been  confined  to  direct- 
current  machinery,  will,  nevertheless,  experience  no  great 
difficulty  in  reading  and  understanding  this  book. 

In  view  of  the  amazing  increase  in  number  and  magni- 
tude of  installations  for  the  transmission  of  power  by  poly- 
phase currents,  this  book  has  been  written  with  special 
reference  to  the  problems  that  belong  to  this  class  of 
engineering  work. 

The  author  desires  to  acknowledge  his  indebtedness  to 
many  electrical  manufacturing  concerns  for  the  use  of 
much  special  and  valuable  information,  and  to  the  electri- 
cal press  for  the  use  of  a  number  of  plates. 

NEW  YORK,  June,  1899. 


196472 


PREFACE   TO    FIFTH    EDITION. 


THE  printing  of  another  edition  of  this  little  work  has 
afforded  the  opportunity  of  bringing  it  up  to  date.  While, 
since  the  appearance  of  the  first  edition,  there  has  been  a 
notable  increase  in  the  size  of  apparatus  units  and  in  the 
development  of  appliances  for  their  control  and  protection, 
the  practical  workings  of  polyphase  systems  in  the  main 
have  not  changed.  The  same  may  be  said  of  the  com- 
mercial application  of  most  polyphase  apparatus,  the 
details  of  construction  of  some  types  of  which,  however, 
have  undergone  considerable  modification.  The  growing 
importance  of  the  single-phase  motor  has  been  duly  con- 
sidered in  the  chapter  on  induction  motors. 

M.  A.  O. 

SCHENECTADY,  N.Y., 

July,  1907. 


TABLE   OF   CONTENTS. 


CHAPTER  PACK 

I.     DEFINITIONS  OF  ALTERNATING-CURRENT  TERMS i 

II.     GENERATORS 21 

III.     GENERATORS  (concluded} 45 

IV.     INDUCTION  MOTORS 84 

V.     INDUCTION  MOTORS  (concluded} 113 

VI.     SYNCHRONOUS  MOTORS 133 

VII.     TRANSFORMERS 157 

VIII.     ROTARY  CONVERTERS 188 

IX.     MOTOR  GENERATORS,  FREQUENCY  CHANGERS,  AND  OTHER 

CONVERTING  APPARATUS 224 

X.     SWITCHBOARDS  AND  STATION  EQUIPMENT 244 

XL     LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION 282 

XII.     TWO-PHASE  SYSTEM 298 

XIII.  THREE-PHASE  SYSTEM  . . .  „ 311 

XIV.  CHOICE  OF  FREQUENCY.  . . 329 

XV.  RELATIVE  WEIGHTS  OF  COPPER  FOR  VARIOUS  SYSTEMS.  335 

XVI.  CALCULATION  OF  TRANSMISSION  LINES 341 

INDEX 363 


OF   THE 

UNIVERSITY 

OF 

UJFORNAJ 


STANDARD   POLYPHASE  APPARATUS 
AND  SYSTEMS. 


CHAPTER   I. 

INTRODUCTOR  Y. 

DEFINITIONS    OF  ALTERNATING-CURRENT 
TERMS. 

Alternating  Currents.  —  On  account  of  the  limitation 
imposed  by  the  space  of  this  book,  mathematical  demon- 
strations of  alternating-current  phenomena  have  been 
omitted  in  the  following  pages,  and  the  chapter  will  be 
found  to  consist  mainly  of  elementary  explanations  and 
statements  which  partake  of  the  nature  of  definitions.  It 
is  hoped  that  these  definitions  will  be  found  useful  in 
aiding  the  uninformed  reader  to  obtain  a  clearer  under- 
standing of  the  principles  underlying  polyphase  appa- 
ratus and  methods.  For  a  more  comprehensive  treatment 
of  alternating-current  phenomena,  the  reader  is  referred  to 
the  many  works  on  the  subject. 

The  alternating-current  generator  was  one  of  the  ear- 
liest applications  of  the  principles  of  induction.  Unlike 
the  current  from  the  direct -current  generator,  which  came 
at  a  later  date,  the  alternating  current  rapidly  changes  its 
value  and  direction,  the  fluctuations  being  periodical. 
Such  a  current  reaches  a  maximum  in  one  sense,  de- 

i 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Q  zero,  rpnt^iq^  and  then  «§<*••*<  a,  »na«imiiiii  in 

the  «dhfi  yiMUfj  as  4^**  as  *fc^  pressure  of  the  generator 
f  oDows  this  variation. 

Assuming,  for  ftimfiiiiij's  sake,  the  case  of  a  two-pole 
..-.—.  ..-.-_  •-.-—  en:  genaafcoi  i:  :^n  :e  sfaarwi]  thai  Eat 
various  popgfinns  of  the  armature  coil  the  number  of  lines 
of  force  enclosed  is  proportional  to  the  cosine  of  the  angle 
through  which  at  any  •y^nt  the  cofl  has  turned,  starting 
from  rh^t  position  in  which  no  electro-motive  force  (E~M~F.) 
is  generated.  The  JIMJJMJ  *••*!••.  value  of  E.M-F.  gener- 
ated at  any  position  of  the  cofl  depends  on  the  rate  of 
change  of  the  number  of  lines  of  force  enclosed.  The 
•amber  of  these  lines  being  proportional  to  the  cosine  of 
the  angle  of  rotation,  this  rate  of  change  will  be  the  same 
as  the  rate  of  change  of  the  cosine.  —  that  is,  proportional 
to  the  sine,  from  which  it  follows  that  the  instantaneous 
value  of  die  generated  EJIJ?.  is  proportional  to  the  sine 
of  the  angle  of  rotation. 

The  variation  of  induced  E.M.F.  being  thus  correctly 
representable  by  the  sine  function,  we  will  draw  the  follow- 


i. 


ing  curve  (Fig.  i)  in  which  the  horizontal  distances  repre- 


les of  rotation  and  in  which  the  vertical  distances 


are  proportional  to  the  sine  of  these  angles. 


ALTERNATING-CURRENT   TERMS.  3 

The  circle  at  the  left  of  the  figure  is  drawn  to  show  the 
method  of  developing  the  sine  curve,  the  point  P  on  the 
circle  being  considered  as  moving  with  a  uniform  velocity, 
and  the  projection  on  the  diameter  of  the  circle  of  the 
radius  OP  being  at  any  instant  proportional  to  the  sine  of 
the  angle  moved  through.  The  vertical  distances  will  also 
represent  the  various  instantaneous  values  of  the  E.M.F. 
corresponding  to  the  various  angular  positions  of  the  cofl, 
and  the  value  at  any  instant  is  equal  to  the  maximum 
value  multiplied  by  the  sine  of  the  angle  instantaneously 
occupied  by  the  coil. 

In  the  two-pole  dynamo  machine  assumed,  a  single 
revolution  of  the  moving  coil  has  been  seen  to  produce  a 
positive  sine  wave  of  E.M.F.  followed  by  a  similar  nega- 
tive wave.  At  the  end  of  the  negative  wave,  and  before 
another  positive  wave  is  started,  the  conditions  are  the 
same  as  at  the  beginning  of  the  first  revolution,  and  addi- 
tional revolutions  will  produce  only  successive  duplicates 
of  the  wave  as  drawn.  The  term  "cycle**  or  "period"  is 
therefore  given  to  the  complete  series  of  £.J/^F.  changes 
as  shown  by  the  curve.  The  name  "frequency"  or  "peri- 
odicity" is  given  to  the  number  of  cycles  that  take  place  in 
•unit  time  of  one  second,  and  we  speak  of  a  frequency  of 
25  cycles  per  second,  or  simply  say  the  frequency  (or 
periodicity)  is  25. 

\Vhile  the  sine  curve  of  l-M/J7.  is  only  approximately 
realized  in  the  average  alternating -current  dynamo,  the 
modern  generator  does  not  usually  depart  widely  from  it, 
and  for  general  purposes  no  sensible  error  is  involved  in 
considering  the  E.M.F.  wave  as  being  sinusoidal. 

On  this  basis,  therefore,  it  remains  to  be  seen  what 
sort  of  reading  will  be  recorded  on  a  voltmeter  at  whose 


4        POLYPHASE  APPARATUS  AND  SYSTEMS. 

terminals  the  E.M.F.  is  constantly  varying  according  to 
the  sine  law.  With  any  commercial  frequency  the  vari- 
ations of  E.M.F.  are  obviously  occurring  far  too  rapidly 
to  permit  the  voltmeter  needle  to  follow  them,  and  the 
needle  will  therefore  take  up  some  kind  of  average  posi- 
tion. An  alternating  current  will  flow  through  the  volt- 
meter, the  alternating  values  of  the  current  corresponding 
to  those  of  the  alternating  E.M.F.  Now,  the  heating  effect 
or  the  dynamometer  effect  of  any  current  (by  which  the 
value  of  the  current  is  measured)  varies  as  the  square  of  its 
value.  Hence  where  the  current  is  not  constant  the  heating 
effect  or  the  dynamometer  effect  will  be  represented  not  by 
the  average  of  the  instantaneous  values  of  the  current  but 
by  the  average  of  the  squares  of  these  values.  This  affords 
the  means  for  determining  that  value  of  E.M.F.  which  if 
steadily  applied  will  have  the  same  heating  effect  cr  the 
same  dynamometer  effect  (i.e.,  which  will  give  the  same 
needle  deflection)  as  that  produced  by  the  alternating 
E.M.F.;  for  what  is  now  to  be  determined  is  only  that 
value  of  steady  E.M.F.  (or  current)  whose  square  is  the 
same  as  the  average  of  the  squares  of  the  several  instan- 
taneous values  of  the  alternating  E.M.F.  The  shape  of 
the  sine  curve  is  such  that,  given  the  maximum  value, 
which  we  will  call  0,  seen  from  Fig.  i  to  be  equal  to  OP 
the  average  of  the  squares  of  the  instantaneous  values  can 
be  shown  to  be  equal  to  a2  -=-2.  The  dynamometer  effect 

being  therefore  — '  the  E.M.F.  (or  current)  producing  this 

effect  is  represented  by  the  square  root  of  this  quantity  or 

a  a 

—p  =  -  —  =  0.7070.  This  is  the  value  which  is  indicated 
V2  1.414 

by  all  alternating   current  measuring  instruments.     It  is 


ALTERNATING-CURRENT   TERMS.  fj 

called  the  virtual  or  mean  effective  value,  and  by  reason 
of  the  process  by  which  it  is  derived  is  frequently  written 
as  \/mean2  and  called  the  square  root  of  the  mean  square, 
or,  briefly,  the  root  mean  square.  As  seen,  it  is  equal  to 
70.7  per  cent  of  the  maximum  value.  This  means  that 
when  a  voltmeter  reads  70.7  volts  the  maximum  peak  of 
E.M.F.  is  100  volts,  or,  on  a  circuit  where  the  instrument 
reads  7070  volts,  the  maximum  rises  to  10,000  volts.  In 
the  case  of  a  high  tension  cable  working  at  7070  volts  (as 
read  on  a  voltmeter)  this  means  that  twice  in  every  cycle 
the  insulation  is  subjected  to  a  potential  higher  by  2930 
volts  than  .that  indicated  by  the  voltmeter,  which  registers 
only  the  effective  E.M.F.  or  root  mean  square.  The  maxi- 
mum is  thus  41.4  per  cent  higher  than  the  mean  effective, 
and  this  is  a  point  which  must  not  be  overlooked,  since 
in  the  case  of  such  a  cable  the  thickness  of  the  insulation 
must  be  proportioned  not  to  the  average  or  to  the  mean 
effective  E.M.F.  but  to  the  maximum  potential  strain. 

This  feature  is  of  all  the  more  importance  when  con- 
sideration is  given  to  the  departures  from  the  true  sine 
form  which  are  found  to  some  extent  in  nearly  all  gener- 
ators. Coincident  with  the  wave  form  corresponding  to 
the  frequency  are  frequently  found  subsidiary  waves 
known  as  harmonics.  These  are  always  odd  number  mul- 
tiples of  the  fundamental  frequency,  that  is,  the  frequency 
of  the  harmonics  is  always  three,  five,  seven,  etc.  times 
that  of  the  fundamental.  In  Fig.  2  is  shown  the  effect  of 
the  third  harmonic  (triple  frequency). 

In  Fig.  2  "i"  is  the  fundamental,  "2"  is  the  triple 
frequency  harmonic,  and  "3"  is  the  resultant  wave.  That 
is,  "3"  would  represent  the  actual  wave  shape  of  an  alter- 
nator in  which  were  present  a  triple  harmonic  of  the  mag- 


6       POLYPHASE  APPARATUS  AND  SYSTEMS. 

nitude  shown.  Obviously  in  such  a  curve  as  "3"  the 
ratio  between  maximum  value  and  root  mean  square  is 
greater  than  in  a  true  sinusoid.  Depending  on  the  value 
of  the  harmonic  relative  to  the  fundamental,  the  distor- 
tion from  the  sine  wave  will  be  of  greater  or  less  importance. 

The  name  amplitude  is  given  to  the  maximum  value  of 
the  wave.  In  Fig.  2  the  amplitude  of  "  i  "  is  a;  of  "  2  "  it  is 
b,  and  the  amplitude  of  the  resultant  wave  "3"  is  c. 

It  is  necessary  to  make  use  of  some  reference  point  in 
designating  the  epoch  at  which  during  the  cycle  some 


Fig.  2. 

event  occurs.  This  reference  point  is  usually  taken  as 
that  at  which  the  positive  wave  makes  its  start.  For 
curve  "i"  in  Fig.  2  this  is  at  o.  It  is  also  at  o  for  curve 
"3."  From  o  to  x,  where  both  these  curves  fall  to  zero 
again  is  180  degrees,  and  x  is  said  to  be  behind  o  by  i8c 
degrees  of  phase.  Similarly,  the  maximum  of  curve  i  is 
reached  after  90  degrees  of  phase  is  passed  through.  If, 
as  in  Fig.  3,  two  waves  are  drawn  as  shown,  wave  b  is 
said  to  be  90  degrees  of  phase  behind  wave  a  because  a 
has  advanced  through  90  degrees  of  its  course  before  b 


ALTERNATING-CURRENT   TERMS.  f 

starts  out.     Two   waves  having  90  degrees  of  phase  dis- 
placement are  often  referred  to  as  being  in  quadrature. 

The  formula  for  the  flow  of  current  in  an  alternating 
system  of  conductors  is,   in  its  general  form,   similar  to 
that  used  for  determining  the 
flow  in  a  direct-current  system. 
It  differs  from  Ohm's  law  only 
in  the  introduction  of   certain 
factors,   which,    however,   may 
become  so  complex  as  to  con- 
ceal the   simple   quantities   of  Fi^  8< 
the  equation  for  current,  resis- 
tance and  E.M.F.     The  value  of  these  factors   depends 
on  three  well-known  properties  of  a  conductor.     These  are: 

1.  Inductance. 

2.  Capacity. 

3.  Virtual  Resistance. 

Inductance.  —  In  the  case  of  a  direct  current  of  con- 
stant value  the  magnetic  field  surrounding  a  circuit  through 
which  a  current  is  flowing,  exerts  no  influence  on  the  cir- 
cuit. In  the  case  of  an  alternating  current  it  is  of  great 
practical  importance,  and  gives  rise  to  a  variety  of  phe- 
nomena. The  magnetic  flux  then  varies  periodically  with, 
and  in  the  same  manner  as,  the  current  and  E.M.F.  The 
setting  up  of  this  magnetic  flux  —  or  lines  of  force,  as  they 
are  sometimes  called  —  produces  an  E.M.F.  in  the  circuit, 
in  opposition  to  the  induced  E.M.F.  This  counter  E.M.F., 
or  E.M.F.  of  self-induction,  is  strongest  when  the  magnetic 
flux  is  changing  most  rapidly;  therefore  arriving  at  a  maxi- 
mum 90  degrees  later  than  the  flux  and  the  current  produc- 
ing the  flux.  The  result  of  this  counter  E.M.F.  is  that,  when 


8      POLYPHASE  APPARATUS  AND  SYSTEMS. 

an  external  E.M.F.  is  applied,  the  current  does  not 
immediately  attain  its  maximum,  and,  when  the  E.M.F.  is 
withdrawn,  the  current  persists  for  a  while.  The  current 
reaches  its  maximum  later  in  point  of  time  than  the 
E.M.F.,  —  i.e.,  is  always  lagging  behind  the  E.M.F.  It 
would  seem  as  if  a  current  of  electricity  possessed  a 
quality  of  the  nature  of  the  inertia  of  matter. 

The  strength  of  this  flux,  or  the  induction  as  Faraday 
called  it,  is  determined  by  the  current.  The  extent  to 
which  a  gixen  flux  affects  a  circuit  in  a  non-magnetic 
medium  —  i.e.,  the  magnitude  of  the  counter  E.M.F.  — 
depends  solely  upon  the  geometry  of  the  circuit.  If  the 
circuit  is  wound  in  a  coil,  or  so  arranged  that  in  the 
periodic  variation  of  the  flux  the  same  lines  of  force 
encircle  more  than  one  portion  of  the  conductor,  the  coun- 
ter E.M.F.  will  be  increased. 

That  constant  quality  of  a  circuit  which  determines  its 
inductive  effects  is  called  inductance.  The  inductance 
may  be  either  self  or  mutual  inductance,  according  as  the 
circuit  is  isolated  or  acted  on  by  an  adjacent  circuit,  also 
carrying  a  current.  Inductance  is  frequently  called  the 
co-efficient  of  induction.  The  symbol  L  is  used  to  desig- 
nate self-inductance,  —  the  unit  of  measurement  of  which 
is  the  henry. 

Capacity.  —  Like  inductance,  the  capacity  of  a  circuit 
depends  upon  its  geometry  and  its  surroundings.  It  is 
the  quality  which  a  conductor  possesses  of  being  able  to 
hold  a  quantity  of  electricity.  A  combination  of  con- 
ductors or  conducting  surfaces,  advantageously  placed  to 
hold  the  greatest  possible  quantity  of  electricity,  is  called 
a  condenser.  All  insulated  lines  act  more  or  less  like 
condensers.  The  charging  or  discharging  current  of  a 


ALTERNATING-CURRENT    TERMS.  9 

condenser  is  greatest  when  the  rate  of  change  of  effective 
pressure  is  greatest;  that  is,  when  the  E.M.F.  is  at  zero  at 
the  moment  of  passing  from  negative  to  positive,  or  vice 
versa.  The  effect  of  capacity,  then,  is  opposite  to  the 
effect  of  inductance,  and  may  neutralize  it,  or  even  over- 
come it,  when  existing  in  the  same  circuit.  In  a  circuit 
having  capacity,  the  current  may  lead  the  E.M.F.  in  phase. 
Fig.  4  shows  the  lead  produced  by  capacity.  The  curve 
V  represents  the  curve  of  E.M.F.,  and  /  the  current 
curve  leading  the  E.M.F.  The  unit  of  measurement  of 
capacity  is  the  farad,  and  is  usually  represented  by  the 
symbol  K. 

Impressed  E.M.F. --The  more  frequently  an  alternat- 
ing current  is  re- 
versed, the  less 
time  is  there  avail- 
able for  it  to  reach 
the  value  it  would 
have  in  a  direct- 
current  system.  Fig  4 

To  drive  this  maxi- 
mum current  through  an  alternating  system  of  conductors 
having  inductance,  requires  a  greater  E.M.F.  than  is  needed 
in  a  direct-current  system  to  produce  this  same  current. 
The  inductance  of  a  circuit,  as  explained,  determines  the 
counter  E.M.F.;  and  it  must  be  overcome  by  an  added 
amount  to  the  E.M.F.  required  to  produce  the  same  cur- 
rent in  a  direct-current  system.  The  name  of  impressed 
E.M.F.  has  been  given  to  this  resultant.  The  counter 
E.M.F.  lags  90  degrees  behind  the  current,  and  is  greatest 
when  the  current  is  reversing  its  sign,  or  when  the  rate  of 
change  of  the  lines  of  force  is  greatest.  The  values  and 


10      POLYPHASE  APPARATUS  AND  SYSTEMS. 

direction  of  the  impressed  E.M.F.  and  its  components  may 
be  considered  in  a  diagram.  In  Fig.  5  the  impressed 
E.M.F.  is  shown  as  the  hypotenuse  of  a  right-angled  tri- 
angle. That  component  of  the  E.M.F.  which  would  drive 
the  same  current  through  a  circuit  without  inductance, 
being  necessarily  in  phase  with  the  current,  is  shown  as 
lagging  behind  the  impressed  E.M.F.  by  an  angle,  <£,  and 
by  a  length  equal  to  its  magnitude.  In  quadrature  with 
this  component  is  the  E.M.F.  of  self-induction,  the  mag- 
nitude of  which  determines  the  length  of  the  line  in  the  dia- 
gram. The  magnitude  of  the 
impressed  E.M.F.  is  then  rea- 
dily  found.  The  name  of  en- 
ergy  E.M.F.  has  been  given 
to  that  component  in  phase 
with  the  current,  and  which  is 

\  Angle  lag,  re.      J .        .       ,    .  ,     . 

effective  in  doing  any  work  in 


IR  Energy  EM F 

a  circuit.     As  all  the  quanti- 
ties in  the  diagram  must  follow 

the  law  of  simple  harmonic  motion,  the  curve  of  self-induc- 
tive E.M.F.  will  be  shown  in  the  same  way  as  the  curve  of 
impressed  E.M.F.  The  effect  of  this  inductive  component, 
in  increasing  the  impressed  volts  needed  to  cause  a  given 
current  to  flow,  is  shown  in  Fig.  6.  The  curve  RI  repre- 
sents the  energy  component  of  the  impressed  E.M.F. 
which  would  drive  the  current  if  there  were  no  induc- 
tance. It  is  equal  in  value  to  the  product  of  the  current 
and  the  resistance.  In  quadrature  with  it,  is  the  E.M.F. 
required  to  overcome  self  induction,  designated  by  the 
curve  pLI,  p  being  equal  to  2  TrN,  where  N  is  the  number 
of  complete  cycles  per  second,  and  L  the  inductance.  This 
is  the  component  required  to  offset  the  effect  of  the  indue- 


ALTERNATING-CURRENT   TERMS. 


II 


tance.     By  adding  the  ordinates   of  the  two  curves,   we 
obtain  a  third  curve,  F,  also  following  the  sine-curve  law. 


v 

Fig.  6. 

This  is  the  curve  of  the  impressed  E.M.F.  required  to  pro- 
duce the  given  current  in  this  particular  circuit. 

Impedance ;  Reactance.  —  Impedance  is  the  total  opposi- 
tion in  a  circuit  to  the  flow  of  current.  It  determines  the 
maximum  current  that  can 
flow  with  a  given  impressed 
E.M.F.  It  is  made  up  of 
a  resistance  component  and 
another  component  to  which 
the  name  of  reactance  has 
been  given.  The  relations 
of  resistance,  reactance,  and 
impedance  are  shown  in  Fig. 
7.  As  there  may  be  energy 

losses  external  to  a  circuit,  and  yet  dependent  on  that  cir- 
cuit, which  require  a  flow  of  current  that  cannot  be  deter- 
mined by  a  calculation  based  upon  the  ohmic  resistance 
alone,  it  is  not  correct  to  designate  the  resistance  com- 


R-Resistance 
Fig.  7. 


12 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


/?- Resistance 


ponent  as  the  ohmic  resistance.  Such  losses  are  those 
due  to  hysteresis  in  transformers  and  iron  cores.  This 
component  of  impedance  is  termed  energy  resistance,  and 
the  other,  reactance,  which  has  sometimes  been  called  the 
inductive  resistance. 

Reactance  is  the  effect  of  self-induction  expressed  in 
ohms.  It  becomes  prominent  in  lines  of  large  cross 
section.  The  relative  value  of  reactance  to  resistance  can 
be  reduced  by  selecting  a  number  of  conductors  of  small 
areas  having  a  combined  equal  resistance.  For  instance, 

when  for  one  No.  ooo 
wire  two  No.  i  wires  are 
substituted,  the  resistance 
will  remain  the  same,  but 
the  reactance  will  be  al- 
most halved. 

When  capacity  is  intro- 
duced into  the  circuit,  the 
current  may  lead  in  phase.  Fig.  8  illustrates  the  effect  of 
capacity  on  the  circuit.  The  reactance  due  to  capacity,  or 
condensance  as  it  is  designated,  acts  in  the  opposite  direc- 
tion to  the  reactance  of  inductance.  The  impedance  in 
Fig.  8  is  the  resultant  of  the  resistance  and  the  capacity 
reactance  or  condensance.  When  capacity  and  inductance 
are  both  present,  the  impedance  is  the  resultant  of  the  re- 
sistance component  and  a  component  equal  to  the  differ- 
ence between  the  numerical  values  of  the  condensance  and 
reactance.  In  Fig.  9  the  reactance  is  laid  off  above  the 
line  of  resistance  and  in  quadrature  with  it.  The  condens- 
ance acting  in  opposing  direction  is  represented  as  having 
a  greater  numerical  value.  The  resultant  impedance  is 
readily  found.  When  the  inductance  is  equal  to  the  con- 


Fig.  8. 


UNIVERSITY 

OF 


ALTERNATING-CURRENT   TERMS.  13 

densance,  the  current  is  in  phase  with  the  impressed  E.M.F. 
and  follows  Ohm's  law. 

In  aerial  conductors  of  low  resistance,  the  reactance  is 
often  prominent,  and  the  distribution  of  E.M.F.  may  be 
seriously  affected  by  it.  It  becomes  important,  then,  in 
selecting  conductors  for  transmission  lines,  that  those  of 
large  cross-section,  and  correspondingly  low  resistance,  be 
avoided  as  much  as  possible,  except  in  special  cases,  as, 
for  instance,  in  a  rotary  converter  supplied  by  its  own  set 
of  conductors,  where  some 
reactance  may  be  desirable. 


Virtual     Resistance.  —  If 


the  cross  section  of  a  con- 
ductor carrying  an  alter- 
nating current  is  resolved 
into  many  elements,  it  will  Fi  9 

be  seen  that  the  internal 
portions  are  subject  to  greater  inductive  effects  than  the 
elements  nearer  the  surface.  The  outer  streams  of  cur- 
rent suffer  less  opposition,  and  reach  a  maximum  sooner 
than  those  centrally  located.  In  large  conductors,  carry- 
ing heavy  currents  of  very  high  frequency,  there  may  not 
only  be  no  current  flowing  in  the  central  portion  of  the 
conductor,  but  a  condition  may  exist  where  a  current  will 
flow  in  the  opposite  direction.  The  central  core  is  then 
not  only  valueless  as  a  conductor,  but  had  better  be 
omitted. 

As  a  result  of  the  reduction  of  the  effective  cross  section 
of  a  conductor  carrying  an  alternating  current,  the  resis- 
tance is  increased,  and  slightly  less  current  will  flow  than 
would  if  the  specific  resistance  and  the  inductance  of  the 
wire  are  alone  considered.  This  increment  of  resistance 


14      POLYPHASE  APPARATUS  AND  SYSTEMS. 

of  a  conductor  is  called  its  virtual  resistance.  The  phe- 
nomenon is  also  called  the  skin  effect. 

The  best  shape  for  conductors  of  large  cross  section,  carry- 
ing heavy  alternating  currents,  is  that  of  a  tube  or  flat  strip. 

In  common  practice  the  sizes  of  wire  and  the  rapidity 
of  current  reversals  are  not  such  as  appreciably  to  produce 
this  effect.  The  ratio  of  the  resistance  of  a  conductor 
carrying  an  alternating  current,  to  its  resistance  when  a 
direct  current  is  flowing,  can  be  readily  computed  for  dif- 
ferent sizes  of  conductors  and  reversals  of  current. 

In  Fig.  10  the  ordinates  represent  the  product  of  area 
and  cycles  per  second.  Corresponding  factors  for  the 
virtual  or  apparent  resistance  of  cylindrical  copper  con- 
ductors are  read  off  the  horizontal  scale.  The  ordinate 
for  the  factor  for  a  conductor  of  any  other  non-magnetic 
metal  is  the  product  of  the  ratio  of  its  conductivity  to  that 
of  copper,  and  the  sectional  area  times  the  frequency.  In 
the  case  of  magnetic  materials,  especially  iron,  the  factor 
for  the  virtual  resistance  is  greater  than  that  in  the  curve. 

Energy  in  a  Circuit.  —  The  work  done  in  a  circuit  will 
always  be  some  product  of  the  current  and  the  quantities 
in  phase  with  it.  In  a  direct-current  system  the  product 
of  measured  volts  and  amperes  will  give  the  energy  of  the 
circuit.  In  an  alternating-current  system  the  product  of 
the  measured  amperes,  and  the  component  of  impressed 
E.M.F.  in  phase  with  the  current,  —  i.e.,  the  energy 
E.M.F.,  —  will  give  the  energy.  The  component  of  E.M.F. 
in  quadrature  with  the  current  —  i.e.,  the  inductive  com- 
ponent —  drives  a  wattless  current,  and  consequently  adds 
nothing  to  the  energy  of  the  circuit.  The  product  of 
the  impressed  E.M.F.  and  the  current  gives  only  the 
apparent  watts  of  the  circuit.  The  error  in  calculating 


ALTERNATING-CURRENT   TERMS.  15 

the  power  by  the  product  of  measured  amperes  and  volts 
will  depend  upon  the  extent  of  the  displacement  in  phase 
of  the  impressed  E.M.F.,  and  the  current,  or  the  angle  of 
the  lag  or  lead,  usually  denoted  as  </>.  The  energy  in  the 


yu 
85 
80 

/ 

. 

/ 

$  70 
Q-  65 

3-60 

x  55 
{» 

o  45 

C    IK 

/ 

' 

/ 

* 

/ 

/ 

' 

/\ 

/ 

/ 

/ 

/ 

•3 

c  25 

/ 

K 

1   15 

10 

/ 

/ 

7 

5 

1.00      1.04       1.08      \Xl       1.16        12       I2E      1.24-       12 

Factor  for  Virtual  Resistance. 
Fig.  10. 

circuit  can  be  found  by  multiplying  the   product  of  volts 
and  amperes  by  the  cosine  of  this  angle. 

Power  Factor  —  Induction  Factor.  —  The  ratio  of  the  true 
watts  in  the  circuit,  as  measured  by  an  indicating  watt- 
meter, to  the  apparent  watts,  —  the  volt-amperes,—  is  called 
the  power  factor.  The  value  of  power  factor  is  useful  in 
determining  the  true  energy  in  a  circuit  when  the  apparent 


l6      POLYPHASE  APPARATUS  AND  SYSTEMS. 

energy  is  known,  the  resistance  when  the  impedance  is 
known,  the  energy  Volts  when  the  total  impressed  volts 
are  given,  and  the  energy  current  when  the  total  current 
is  known. 

The  quantities  in  quadrature  with  the  energy  values  of 
current  and  E.M.F.,  and  with  the  resistance,  may  be  deter- 
mined in  the  same  way,  from  the  resultants  by  a  multiplier 
or  factor,  called  the  induction  factor.  As  the  power  factor 
is  proportional  to  the  energy  components,  and  the  induction 
factor  to  the  components  in  quadrature  with  them,  it  follows 
that  the  former  must  be  numerically  equal  to  the  cosine,  and 
the  latter  to  the  sine  of  the  angle  of  phase  displacement. 
Accordingly,  a  table  of  cosines  and  sines  for  air  angles  will 
give  the  corresponding  power  and  induction  factors. 

Wattless  Current.  —  The  component  of  the  total  current 
in  quadrature  with  the  energy  current  is  called  the  watt- 
less current.  Tt  should  be  understood  that  the  current  and 
other  quantities  of  a  circuit  are  resolved  into  components 
only  for  the  sake  of  a  better  understanding  of  the  phe- 
nomena taking  place  in  the  circuit.  There  is  actually  but 
one  current  flowing,  as  there  is  but  one  E.M.F.,  in  any  one 
part  of  a  circuit.  The  presence  of  reactance,  either  in  the 
transmission  circuit  or  in  the  apparatus  connected  to  it, 
increases  the  lag-angle,  and  consequently  the  wattless 
current.  This  component  does  no  work  in  a  circuit,  but 
increases  the  total  current,  and  thereby  the  heating  of 
conductors.  The  wattless  current  required  to  balance  the 
reactance  may  become  sufficiently  great  to  practically  tax 
the  full  capacity  of  generators  and  of  conductors,  although 
very  little  energy  is  being  generated  or  transmitted.  If  it 
were  possible  to  have  conductors  without  resistance,  a 
true  wattless  current  could  then,  in  fact,  actually  exist  in 


ALTERNATING-CURRENT   TERMS.  I? 

an  alternating-current  circuit.  In  such  a  case  the  total 
current  would  be  in  quadrature  with  the  impressed  E.M.F., 
and  the  circuit  would  give  back  as  much  energy  as  it 
received,  the  sum  being  zero. 

Relative  Values.  —  Designating  the  current  as  /,  resis- 
tance as  R,  reactance  as  S,  and  impedance  as  Uy  from 
what  has  preceded,  the  following  relations  will  be  under- 
stood : 

1.  The  reactance  |         Induction  E.M.F.  consumed  in  line 

of  a  line,  S,   -j  I 

™      .  T       Impressed  E.M.F,  consumed  in  line 

2.  The  impedance,  £/,=—•  — - —  —  • 

3.  The  energy  component  of  E.M.F.  consumed  by  the  resis- 

tance, R,  of  a  conductor  is  IR,  and  is  in  phase  with  the 
current. 

4.  The    inductive    component   of    E.M.F.    consumed   by   the 

reactance,  S,  of  a  conductor  is  SS,  and  is  in  quadrature 
with  the  current. 

5.  The  impressed  E.M.F.  consumed  by  the  impedance, U,  of  a 

conductor,  is  IU. 

5.    The  energy  loss  in  a  conductor  is  I*R,  and  depends  on  the 
current  and  resistance  only. 

Voltage  Drop  Dependent  on  Load  Characteristic.  —  The 

E.M.F.  consumed  by  the  impedance,  IUt  does  not  represent 
the  voltage  drop  in  a  conductor,  as  it  is  usually  out  of  phase 
with  the  impressed  E.M.F.  as  well  as  with  the  current. 
This  voltage  drop,  as  will  be  shown,  can  be  anything  be- 
tween IR  and  IU.  It  will  depend  upon  the  difference  in 
phase  between  the  current  and  the  impressed  E.M.F.,  or 
the  lag  angle,  and  can  be  determined  when  the  power 
factor  is  known.  In  Figs,  n  to  16,  let  OE'  be  the  E.M.F. 
at  the  receiving  end  of  a  transmission  line.  For  various 


1 8      POLYPHASE  APPARATUS  AND  SYSTEMS, 

power  factors  at  the  receiving  end  of  the  line,  there  will  be 
corresponding  phase  differences,  <£.  Let  OI  be  the  cur- 
rent out  of  phase  with  the 
E.M.F.  by  4>.  IU,  IR, 
and  IS  have  the  relations 
heretofore  assigned  to 
them,  IR  being  in  phase 
with  OI,  and  76*  in  quad- 
rature with  OI.  Where 

<J>=9O°  ~ ,-flF 

these  quantities  are  small 


relatively  to  the  impressed 
Fig.  11.  E.M.F.,  as   they   usually 

are  in  practice,  the  drop 

of  voltage  is  I  A,  equal  to  OE  —  OEf,  OE  being  equal  to 
the  generator  voltage,  A  the  apparent  resistance  of  the  line. 
Assume  a  given  E.M.F.  at  the  end  of  the  line,  and  a 
constant  resistance  and  reac- 
tance. If  the  phase  displace- 
ment </>,  or  what  is  the  same, 
if  the  power  factor  of  the 
receiving  system,  is  varied, 
the  triangle  of  electromotive 
forces  will  revolve  around  Ef 
as  a  center.   The  projection  of       /   0=60° 

IU,  or  its  components,  upon v 

the  E.M.F.  will  give  the  vol-  K  E'     U        E 

tage  drop.     With  a  lag  angle 

of  90°  (Fig.  n),  the  drop  of  voltage  is  due  to  the  reac- 
tance alone.  As  the  lag  angle  decreases,  the  drop  I A  be- 
comes less  than  the  impressed  E.M.F.  consumed  in  the  line 
IU  until  it  reaches  60°  (Fig.  12),  when  with  the  given 
values  of  IU  and  IS  the  drop  is  seen  to  be  equal  to  the 


ALTERNATING-CURRENT  TERMS. 


impedance  /£/,  and  has  the  greatest  value  it  can  have. 
As  the  phase  displacement  grows  less,  the  effect  of  the 
reactance  decreases  until  </>=  o  (Fig.  13),  when  the  drop 
is  due  to  resistance  alone,  a  case  of  a  non-inductive  load. 

If  capacity   effect   is 
now  introduced  into  the    /-, 
line  by  the  use  of  long 
cables,  over  excited  syn- 


Fig. 13. 


chronous,  motors,  over 
excited  rotary  convert- 
ers, or  of  condensers,  the  phase  displacement  $  becomes 
negative.  Up  to  30  degrees  the  projection  of  the  reactance 
is  in  opposition  to  the  projection  of  the  impedance,  i.e., 

negative    (Fig.    14), 

and  as  a  result  the 

drop  I A  is  less  than 
the  resistance  drop. 


Fig.  14. 


Finally,  at  30  de- 
grees (Fig.  15)  there 
is  no  drop  of  voltage  in  the  line;  for  the  reactance  raises 
the  voltage  as  much  as  the  resistance  lowers  it,  and  the 
line  apparently  has  no  resistance.  As  the  phase  displace- 
ment increases,  the 
voltage  at  the  receiv- 
ing end  becomes 
higher  than  the  gen- 
erator E.M.F.,  due 
to  the  predominat- 
ing effect  of  the  con- 
densance  over  the  resistance.  This  is  the  greatest  at  90 
degrees  (Fig.  16).  For  the  sake  of  simplicity  we  have 
assumed  in  the  foregoing  that  the  projection  of  E  deter- 


Fig.  15. 


20      POLYPHASE  APPARATUS  AND  SYSTEMS. 

mines  the  apparent  resistance.  This  is  not  strictly  accu- 
rate, but  in  practice  the  error  involved  will  be  found  to 
be  insignificant. 

Frequency.  —  As   previously   stated,   the   term  cycle   or 
period  is  given  to  the  complete  series  of  E.M.F.  changes  as 

shown  by  the  curve  in 
Fig.  i,  and  the  name 
frequency  or  periodi- 
city is  given  to  the  num- 
ber of  cycles  that  take 
place  in  one  second. 

Fi     16  The    same    nomencla- 

ture is  used  with  any 
other  alternating  quantities,  such  as  current,  flux,  etc. 

In  a  bipolar  generator  every  revolution  of  the  armature 
corresponds  to  one  cycle.  In  multipolar  generators  there 
will  be  as  many  cycles  for  every  revolution  as  there  are 
pairs  of  poles.  In  a  twenty-four  pole  generator  of  300 
R.P.M.  there  will  therefore  be  five  revolutions  per  second 
and  twelve  cycles  per  revolution,  equalling  sixty  cycles  per 
second.  Frequency  is  sometimes  stated  in  alternations 
per  minute.  As  there  are  two  alternations  to  each  cycle 
the  number  of  alternations  per  minute  will  be  2  X  60  =  120 
times  the  number  of  cycles  per  second,  and  the  generator 
in  question  would  give  60  X  120  =  7200  alternations. 
From  the  above  we  have :  - 

,       Poles  X  R.P.M. 

a)  Frequency    in    cycles     per    second  = 

b)  Frequency  in  alternations  per  minute  =  Poles  X  R.P.M. 


GENERATORS.  21 


CHAPTER   II. 
GENERATORS. 

As  an  elementary  form  of  polyphase  generator  we  may 
take  the  case  of  two  single-phase  alternators  coupled 
together  on  one  shaft  in  such  a  manner  that  the  electro- 
motive forces  at  the  terminals  of  the  respective  armatures 
arrive  at  a.  maximum  90  degrees,  or  one-quarter  of  a 
period,  apart.  The  currents  from  such  a  machine  are  said 
to  have  a  two-phase  relationship.  An  arrangement  of 
three  such  armatures  with  similar  coils  one-third  of  a  pole 
arc,  or  120  electrical  degrees,  apart,  will  generate  three- 
phase  currents.  Fig.  17  illustrates  the  armature  connec- 


Fig.  17. 

tions  of  such  a  three-phase  unit  made  up  of  three  single- 
phase  alternators  arranged  in  the  manner  indicated. 

The  combination  of  two  or  more  independent  alterna- 
tors forming  one  polyphase  unit  facilitates  the  regulation 
of  the  circuits  in  case  of  unbalancing  since  the  fields  (not 
shown  in  the  diagram)  in  which  the  respective  armatures 
revolve,  are  separate  and  may  be  individually  adjusted. 
Such  a  form  of  polyphase  generator  is  not  commercially 


22      POLYPHASE  APPARATUS  AND  SYSTEMS. 

manufactured  as  it  is  naturally  expensive.  Being  made 
of  smaller  machines,  the  cost  is  greater  than  that  of  a  single 
unit  of  the  same  output.  In  a  machine  for  the  generation 
of  polyphase  currents,  therefore,  the  several  windings  for 
the  different  phases  are  wound  upon  the  same  armature, 
the  coils  being  angularly  displaced  to  obtain  the  neces- 
sary phase  relation.  Since  the  flux  which  generates  the 
voltage  in  the  several  phases  is  not  a  maximum  in  each 
phase  at  the  same  instant,  the  total  quantity  of  armature 
iron  may  be  considered  as  usefully  employed  by  each 
phase  successively.  In  this  way  the  same  quantity  of  iron  is 
more  economically  employed  in  a  polyphase  than  in  a  single- 
phase  armature.  As  a  result,  polyphase  generators  are 
smaller  and  cheaper  than  single-phase  generators  of  the 
same  capacity.  With  the  type  of  construction  commonly 
employed,  therefore,  the  polyphase  generator  has  but  one 
field  and  one  armature,  with  as  many  sets  of  windings  as 
there  are  phases.  Irregularities  in  the  voltage  of  the  dif- 
ferent phases,  if  any  exist,  must  be  overcome  in  some  other 
manner  than  by  the  variation  of  the  field  strength.  In 
some  inductor  types  of  generators  this  regulation  is  ob- 
tained by  varying  the  number  of  armature  turns  in  the 
unbalanced  phase. 

In  general,  the  principles  of  construction  and  operation 
of  single-phase  generators  apply  equally  well  to  polyphase 
machines. 

Revolving  Armature  Type.  —  A  type  of  alternating  cur- 
rent generator  at  one  time  commonly  employed  is  that  in 
which  the  armature  is  the  moving  member.  A  typical 
machine  of  this  class  of  400  kilowatts  capacity,  is  well 
illustrated  in  Fig.  18.  This  type  resulted  naturally  from 
the  accepted  and  necessary  form  of  construction  used  in 


GENERATORS. 


direct  current  machines  in  which  the  armature  is  made  the 
revolving  member  because  otherwise  the  current  collecting 


Fig.  18. 


brushes  would  have  to  revolve  also,  an  arrangement  involv- 
ing difficult  and  almost  prohibitive  operating  conditions. 


24      POLYPHASE  APPARATUS  AND  SYSTEMS. 

Revolving  Field  Type.  —  So  far  as  concerns,  however, 
the  generation  of  E.M.F.,  all  that  is  required  is  relative 
motion  between  armature  and  field,  and  the  same  effect  is 
produced  by  keeping  the  armature  stationary  and  revolv- 
ing the  field  as  results  from  the  reverse  operation. 


Fig.  19. 

In  the  construction  of  alternating  current  generators  it 
has  in  general  been  found  more  desirable  to  make  the 
armature  the  stationary  member,  and  most  alternators  are 
now  built  in  this  type.  The  advantages  are  principally 
the  avoidance  of  moving  contacts  except  such  as  are 
required  for  the  moderate  and  low  potential  currents 
required  for  excitation.  The  stationary  armature  machine 


UNIVERSITY 

OF 


GENERATORS.  25 

is  also  more  readily  adapted  to  the  generation  of  currents 
at  high  potential  as  it  is  easier  to  insulate  and  support  the 
armature  coils  securely  where  they  are  stationary  than 
where  they  are  subjected  to  the  centrifugal  and  vibrational 
strains  which  they  would  otherwise  have  to  withstand. 


Fig.  20. 


The  revolving  field  type  of  generator  shown  in  Figs.  19, 
20  and  21  is  one  of  the  number  of  forms  of  the  stationary 
armature  machine.  The  generator  illustrated  is  of  100 
kilowatt  capacity  at  2300  volts.  It  has  eight  poles,  runs 
at  900  revolutions  per  minute,  and  consequently  delivers 
current  at  a  frequency  of  60  cycles. 


26      POLYPHASE  APPARATUS  AND  SYSTEMS. 

Fig.  21  shows  the  revolving  field  of  this  generator.  It 
consists  of  a  heavy  cast  steel  center,  to  which  are  keyed 
the  pole  pieces  projecting  radially  outward.  These  are 
built  up  of  laminated  sheet  iron  in  order  to  minimize  eddy 
current  losses.  The  coils  are  wound  on  spools  placed  on 
the  poles  and  held  in  place  by  the  pole  tips.  In  the  illus- 
tration, metal  pieces,  or  bridges,  are  shown  between  the 
pole  tips.  Bridges  of  this  or  equivalent  form  are  sometimes 
desirable  to  facilitate  parallel  operation  of  generators. 
Bridges  are  also  sometimes  used  on  synchronous  motors 


Fig.  21. 

to  prevent  the   pulsation  which  is  likely  to  occur  under 
unfavorable  conditions. 

The  field  coils  are  preferably  made  of  a  single  spiral  of 
strip  copper,  wound  on  edge,  as  illustrated  in  Fig.  22, 
(which  shows  the  construction  of  the  field  spools  of  a  750 
kilowatt  generator),  the  field  poles  being  in  this  case 
secured  by  bolts  to  the  periphery  of  the  field  ring. 
Mechanical  difficulties  in  forming  this  edgewise  winding 
prevent  the  use  of  very  thin  strip,  and  as  a  result  the  total 
resistance  of  a  strip  winding  of  this  kind  may  be  so  low  in 
a  generator  of  small  capacity  having  few  poles  that  a  low 


GENERATORS.  2? 

voltage  for  excitation  is  desirable  in  order  to  prevent 
excessive  waste  of  energy  in  the  field  rheostat  While  a 
potential  of  about  125  volts  is  commonly  in  use  for  the 
excitation  of  alternators,  it  may,  under  the  conditions 
described,  be  advisable  to  excite  from  a  circuit  of  lower 
potential,  as,  say,  60  volts.  When,  as  shown  in  the  illus- 
tration, the  alternator  is  equipped  with  its  own  exciter, 
the  voltage  of  the  exciter  may  be  chosen  at  any  value 
which  is  desired,  and  for  very  small  machihes  such  ex- 


Fig.  22. 


citers  are  frequently  wound  for  60  volts.  Current  from 
the  exciters  is  often  used  for  other  purposes  than  for 
excitation,  as  for  station  lighting,  and  in  such  cases  it 
is  desirable  to  adhere  to  about  125  volts.  In  such  cases 
small  alternators  with  few  poles  have  the  field  coils  wound 
of  wire  so  as  to  make  the  size  of  the  field  conductor  appro- 
priate to  the  excitation  voltage.  The  strip  winding  is, 
however,  simplest  and  best,  as  it  is  more  easily  insulated 
and  dissipates  the  heat  more  readily.  Direct  current  for 


28 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


excitation  is  carried  to  the  field  windings  by  means  of  two 
cast  iron  or  copper  collector  rings  equipped  with  carbon 
brushes,  requiring  practically  no  attention  in  operation. 

The  stationary  armature  shown  in  Fig.  20  is  of  the  iron- 
clad type  and  is  built  up  of  laminations  slotted  to  admit 
the  coils. 


Fig.  23. 


These  are  usually  machine- wound  and  held  firmly  in 
place  by  wedges  of  seasoned  wood.  Any  injury  to  the  insula- 
tion from  lightning  or  other  causes  is  usually  limited  to 
one  or  to  a  few  adjacent  coils,  which  can  easily  be  replaced 
without  disturbing  the  rest  of  the  winding. 


GENERATORS.  29 

Fig.  23  shows  a  typical  Westinghouse  belted  alternator 
of  200  kilowatt  capacity.  It  has  24  poles  and  runs  at 
300  revolutions  per  minute,  giving  60  cycles. 

All  of  the  standard  belted  polyphase  generators  of  the 
revolving  field  type  conform  to  the  general  lines  of  the 
generators  shown  in  Figs.  19  and  23.  Generators  of  an 
output  greater  than  200  kilowatts  are  usually  provided  with 
a  third,  or  outboard,  bearing,  to  sustain  the  weight  of  the 
pulley  and  strain  of  the  belt.  Generators  of  500  kilowatts 
and  over  are  almost  invariably  built  for  direct  connection 
to  either  engine  or  water  wheel.  If  built  for  connection  to 
the  former  the  base  is  ordinarily  omitted,  while  gen- 
erators for  coupling  to  water  wheels  are  commonly  pro- 
vided with  base  and  two  bearings,  and  in  small  sizes  are 
self-contained  as  a  rule,  the  base  and  two  bearings  either 
comprising  one  casting  or  consisting  of  two  bearing  stand- 
ards dowelled  and  bolted  to  the  cast  iron  base,  or  following 
the  construction  shown  in  Fig.  19,  having  the  bearings 
supported  by  the  end  shields,  a  construction  which  saves 
in  floor  space,  weight  and  cost.  Whenever  possible,  a 
generator,  irrespective  of  its  size,  should  be  direct  con- 
nected on  account  of  saving  of  space  and  of  belt  losses, 
providing  the  increase  in  cost  incident  to  slower  speeds,  if 
such  are  necessary,  does  not  offset  the  advantages  men- 
tioned. 

Fig.  24  shows  the  construction  of  the  stationary  arma- 
ture of  the  machine  of  which  the  field  coils  are  shown  in 
Fig.  22.  The  revolving  field  acts  like  a  fan,  forcing  the 
air  outwardly  through  the  openings,  or  air  ducts,  between 
the  armature  laminations. 

The  stationary  armature  consists  of  a  circular  cast  iron 
frame,  or  spider,  inside  of  which  are  dovetailed  sheet  steel 


30      POLYPHASE  APPARATUS  AND  SYSTEMS. 

discs,  with  slots  to  receive  the  coils.  Ventilating  spaces,  or 
ducts,  are  left  between  the  laminations,  through  which  the 
air  flows  rapidly  when  the  generator  is  running. 


Fig.  24. 


Fig.  25  is  a  sectional  view  of  the  field  and  armature  of  a 
typical  revolving  field  three  phase  generator.  The  rela- 
tion of  the  magnetic  circuit  to  the  armature  coils  is  clearly 
shown. 


GENERATORS.  3! 

The  generator  shown  in  Fig.  26  is  one  which  has  recently 
been  put  in  service  at  the  station  of  a  water  power  com- 
pany in  Mexico,  and  is  a  good  example  of  a  modern,  high 
speed,  three-phase  alternator  of  large  capacity.  It  has  14 
poles  and  runs  at  514  revolutions  per  minute,  giving  a 
frequency  of  60  cycles.  The  driving  power  is  supplied  by 
a  high  head  turbine  direct  coupled  to  the  shaft,  the  half 


Fig.  25. 


coupling  on  the  generator  end  being  forged  integral  with 
the  shaft.  The  machine  is  wound  for  2300  volts  and 
delivers  752  amperes  in  each  phase  when  generating  its 
normal  output  of  3000  kilowatts.  The  commercial  effi- 
ciency of  this  machine  at  full  load  is  97.3  per  cent.  The 
regulation  at  non-inductive  load  is  5.5  per  cent.  The  re- 
volving field  is  mounted  nearer  to  one  than  to  the  other 
bearing  so  that  when  the  armature  is  slid  along  the 


32  POLYPHASE   APPARATUS   AND    SYSTEMS. 


GENERATORS.  33 

machined  surface  of  the  base  toward  the  observer,  the 
windings  are  made  accessible  for  inspection  or  repair. 

Another  form  of  the  stationary  armature  type  of  gener- 
ator is  one  in  which  the  field  winding  is  a  single  coil.  The 
exciting  coil  is  wound  on  a  bobbin  occupying  a  channel  on 
the  periphery  of  a  cast-iron  wheel.  Two  steel  rims  are 
bolted  to  this,  the  laminations  being  formed  into  poles. 
This  is  one  of  the  original  forms  of  polyphase  generator; 
and  this  construction,  which  has  considerable  merit,  was 
adopted  in  the  early  days  of  power  transmission  apparatus 
by  European  manufacturers. 

Inductor  Type.  —  Another  modification  of  the  stationary 
armature  type  is  the  inductor  machine,  manufactured  to 
some  extent  abroad,  and  in  this  country  chiefly  by  the 
Stanley  Electric  Company  and  by  the  Warren  Electric 
Manufacturing  Company.  The  distinguishing  character- 
istic of  this  type  is  that  any  one  set  of  armature  coils,  or 
portion  of  the  armature  conductors,  is  subjected  to  a  mag- 
netic flux  of  one  polarity  only.  The  magnetism  fluctuates 
from  zero  to  maximum  and  back  again  and  does  not 
reverse  its  sign.  Most  generators  of  this  type  have  both 
fixed  armature  and  fixed^  field  windings,  the  only  moving 
part  being  the  inductor  —  a  laminated  iron  core  with  polar 
projections.  The  exciting  winding,  wound  into  an  annular 
coil,  is  sometimes  placed  centrally  on  the  internal  surface 
of  the  armature  spider,  embracing  the  revolving  element, 
as  in  the  Stanley  machine.  This  is  usually  a  ring  of  iron 
with  a  double  row  of  laminated  polar  projections.  In 
some  machines,  such  as  those  manufactured  by  the  Warren 
Company,  the  armature  has  a  single  set  of  coils,  and  the 
inductor  is  provided  with  a  single  row  of  laminations. 
The  annular  exciting  coil  may  be  part  of  the  revolving 
element,  and  revolve  with  it. 


34  POLYPHASE   APPARATUS   AND    SYSTEMS. 


GENERATORS. 


35 


Reference  to  Fig.  27  will  show  the  general  arrangement 
of  the  magnetic  circuit  of  the  Stanley  inductor  generator. 
The  annular  field  coil,  F,  is  surrounded  by  the  magnetic 
circuit,  made  up  of  the  laminated  cores  A  A,  the  armature 
yoke  Yv  and  the  laminated  poles  N  and  5,  and  the  field 
yoke  Y2.  The  armature  windings,  consisting  of  two  corn- 


Fig.  28. 

plete  sets,  are  laid  in  grooves  in  the  armature  cores  in  a 
manner  similar  to  the  revolving  field-machine.  It  will  be 
seen  that  the  north  and  south  poles  do  not  alternate,  but 
the  magnetic  flux  simply  pulsates  in  one  direction.  Only 
one-half  of  each  turn  of  the  armature  winding  is  in  an 
active  field  at  one  time,  the  other  half  of  the  coil  being 
between  the  poles  in  an  inactive  field.  The  E.M.F.  gen- 


36      POLYPHASE  APPARATUS  AND  SYSTEMS. 

crated  is  one-half  as  great  as  it  would  be  if  the  polarity  of 
the  flux  were  reversed.  In  order  to  obtain  a  given  E.M.F. 
with  the  inductor  type  of  machine,  either  the  armature 
windings  or  the  total  magnetic  flux  must  be  doubled.  The 
essential  characteristics,  therefore,  of  an  inductor  gener- 
ator are  a  rather  high  density  of  the  magnetic  circuit,  and 
a  short  air  gap,  the  latter  in  order  to  reduce  the  magnetic 
leakage  to  a  minimum.  The  stationary  element  of  the 
Stanley  inductor  machine  consists  of  two  series  windings, 


Fig.  29. 

forming  two  separate  armatures.  The  currents  in  the 
coils  are  usually  in  quadrature  with  each  other,  thus  giving 
a  two-phase  current.  A  three-phase  relationship  can  be 
established  by  means  of  a  symmetrical  three-phase  wind- 
ing, or  by  making  one  set  of  coils  with  0.86  the  number  of 
turns  of  the  other,  and  connecting  the  end  to  the  middle 
of  the  larger  coil.  By  the  theory  of  the  resultant  of  elec- 
tromotive forces,  the  currents  in  the  three  circuits  will  be 
equal,  and  the  impulses  will  follow  one  another  at  inter- 
vals of  1 20  degrees.  Fig.  28  shows  a  typical  Stanley 


GENERATORS. 


37 


inductor  generator  in  course  of  assembly.  Fig.  29  shows 
a  portion  of  a  similar  machine  in  which  the  details  of  the 
armature  and  field  construction  are  seen. 

Fig.  30  shows  a  sectional  view  along  the  shaft  of  an 
inductor  generator  manufactured  by  the  Warren  Electric 


Fig=  30. 

Manufacturing  Company.  A  is  the  frame,  or  spider,  of 
the  stationary  armature,  into  which  are  dovetailed  the 
armature  laminations  C.  L  are  the  armature  coils 
embedded  in  the  iron.  The  revolving  element  is  made 
up  of  the  spider  B  carrying  the  laminated  polar  pro- 
jections D.  F  is  a  single  magnetizing  coil.  The 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


magnetic  circuit  is  from  B,  through  D  to  C,  and  thence 
from  A  to  B.  It  will  be  seen  that  there  are  two  air  gaps, 
one  between  D  and  C,  and  the  other  between  A  and  B. 
H  is  an  auxiliary  magnetizing  coil,  the  purpose  of  which  is 
to  counterbalance  the  magnetic  pull  due  to  the  main  field 
coil  Fj  so  that  there  shall  be  no  unbalanced  pull  in  one 
direction  or  the  other  along  the  shaft.  As  in  all  inductor 
generators,  the  magnetism  pulsates  only,  and  the  revolving 
polar  projections  have  one  polarity. 

Generators  with  stationary  armatures  are  now  wound 
for  pressures  up  to  18,000  volts,  and  there  are  no  insuper- 
able difficulties  to  be  encountered  in  winding  them  for 
even  higher  potentials.  The  tendency,  however,  is  toward 
the  use  on  transmission  lines  of  potentials  higher  than 
machines  can  well  be  designed  to  generate  directly,  and  as 
transmission  potentials  of  60,000  and  80,000  volts  and 
higher  have  either  already  demonstrated  their  commercial 
success  or  are  to  be  attempted  in  the  near  future,  it  is 
probable  that  in  order  to  reduce  the  cost  of  the  line  the 
higher  potentials  will  in  all  cases  be  employed.  As  these 
potentials  are  probably  beyond  the  limit  of  voltage  that 
can  be  economically  generated  direct  the  use  of  set-up  trans- 
formers for  these  extra 
high  potentials  will  con- 
tinue, and  generators  of 
18,000  volts  and  over  will 
be  the  exception. 

Armature  Windings.— 
For  details  of  windings 
of  generator  armatures 

the  reader  is  referred  to  the  comprehensive  works 
on  the  subject.  The  armature  windings  of  polyphase 


GENERATORS.  39 

generators  are  composed  of  two  or  more  groups  of 
single-phase  windings  suitably  connected  to  give  the 
desired  phase  relation.  Fig.  31  shows  the  elements  of  a 
typical  three-phase  winding.  The  armature  windings  of 
the  modern  alternators  are  laid  in  slots  or  grooves  below 
the  surface  of  the  armature  punchings.  The  shape  and 
number  of  the  slots  have  a  material  effect  upon  the  per- 
formance of  the  gener- 
ator. The  old-fashioned 
iron-clad  armature  had 
one  coil  for  each  pair  of 
poles,  laid  in  deep  slots. 
Onaccount  of  this  group- 
ing of  the  conductors  in- 
to a  coil  of  many  turns, 

this  generator  possessed  Fi     32 

high  armature  reaction 
and  consequently  poor  regulation,  and  could  be  short- 
circuited  with  no  bad  effects.  This  construction  is 
sometimes  carried  out  in  those  modern  polyphase  gen- 
erators whose  armatures  have  one  slot  for  each  phase  and 
each  pole,  and  are  called  unitooth  machines.  Thus  the 
armature  of  an  eight-pole,  two-phase  generator  would  have 
eight  coils  and  a  three-phase  generator  of  the  same  number 
of  poles  would  have  twelve  coils.  The  shape  of  the  arma- 
ture punchings  of  a  twelve-pole,  unitooth,  three-phase, 
revolving  armature  generator  is  shown  in  Fig0  32. 
Sometimes  the  laminations  have  circular  holes  instead  of 
slots.  In  this  case  the  armature  conductors  are  threaded 
through  these  holes  by  hand.  As  the  surface  of  the 
armature  (the  holes  being  beneath  the  surface)  is  thus 
continuous,  there  is  little  or  no  tendency  for  eddy  currents  to 


4O      POLYPHASE  APPARATUS  AND  SYSTEMS. 

be  set  up  in  the  faces  of  the  field  poles,  which  can  therefore 
be  cast  solid,  instead  of  being  laminated.  This  is  considered 
an  advantage  by  some  makers,  the  cost  of  the  field  poles 
being  thereby  reduced  and  a  certain  useful  effect,  equivalent 
to  that  secured  by  bridges  between  the  pole  tips,  being 
secured.  With  this  form  of  armature  construction,  how- 
ever, the  difficulty  of  repair  is  enhanced,  as  the  coils  are 

not  readily  removable,  as 
in  the  case  of  open-slot 
machines.  Armature  re- 
action deforming  the 
wave  shape  of  the 
E.M.F.,  and  high  self 
induction,  requiring 
large  exciting  currents 
Fig  33.  at  full  load,  are  often 

characteristic  of  the  uni- 

tooth  winding.  Such  generators  can,  however,  be  designed 
so  as  in  a  great  measure  to  overcome  these  objections. 
It  is  fortunate  that  this  is  so  because  certain  generators 
of  high  potential  are  more  readily  constructed  in  the 
unitooth  design  by  reason  of  the  greater  economy  of 
insulating  space  secured. 

Most  modern  polyphase  armatures  have  two  or  more 
slots  per  pole  per  phase.  The  slots  are  open,  which,  with 
the  distributed  form  of  winding,  gives  a  very  low  induc- 
tance. Fig.  33  shows  the  armature  punchings  of  such  a 
machine.  The  low  inductance,  together  with  lessened 
armature  reaction  which  this  construction  insures,  im- 
proves the  regulation  of  the  machine,  in  other  words, 
reduces  the  increase  of  exciting  current  at  full  load.  Gen- 
erators with  multitooth  armatures  are  in  general  more 


GENERATORS.  4! 

suitable  for  long-distance  transmission  at  high  potential. 
Their  regulation  is  good  and  the  wave  shape  approaches 
a  sine  curve,  the  best  shape  for  this  work,  as  it  reduces 
the  possibility  of  resonance,  or  rise  of  voltage  at  a  distant 
point  in  the  transmission  circuit  above  that  at  the  generator 
end. 

The  various  connections  of  generator  armature  windings 
will  be  found  explained  in  the  chapters  on  polyphase  systems. 

Electromotive  Force.  —  The  drop  in  E.M.F.  at  the  ter- 
minals of  a  direct  current  generator,  as  the  output  is  in- 
creased, is  due  principally  to  the  armature  resistance  and 
armature  reaction.  In  alternators  the  resistance  drop 
(I.R.)  is  generally  not  so  prominent  as  that  due  to  induc- 
tance and  to  armature  reaction.  The  counter  E.M.F.  of 
self-induction  lowers  the  terminal  pressure  on  the  usual 
loads  with  lagging  component,  while  armature  reaction, 
by  opposing  its  flux  to  the  field  magnetism,  reduces  the 
effective  number  of  lines  of  force  passing  through  the  arma- 
ture conductors  with  the  like  result. 

The  inductance  of  unitooth  armatures  can  be  lessened 
by  widening  the  opening  of  the  slots,  which,  at  the  same 
time,  increases  the 

resistance    to     the  f        ^\c  B 

magnetic  flux,   i.e., 

the    reluctance    of  

the    air    gap.      As       ^/  /     \  \.  Yj/ 

inductance     varies        ^^^^s^  \     ^^^^^f 

directly    with    the  \^        / 

square  of  the  num-  Fi     84 

ber   of  turns,    this 

property  can  be  much  reduced  without  sacrificing  efficiency 

or  increasing  the  cost  of  the  generator,  by  using  fewer  turns 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


per  slot  and  more  slots  —  in  other  words,  the  distributed 
form  of  winding. 


Fig.  35. 

Armature  reaction  is  greatest  when  the  load  is  inductive, 
as  then  the  phase  displacement  between  current  and  E.M.F. 


Fig.  36. 

brings   the   maximum   armature   magnetism   in   the   most 
favorable  position  for  reacting  on  the  field.     The  distrib- 


GENERATORS.  4:3 

uted  winding  minimizes  the  effect  of  armature  reaction, 
because  tr^e  separate  portions  of  a  coil  constituting  one 
phase  do  not  occupy  the  same  angular  position  with  refer- 
ence to  the  pole,  and  therefore  the  separate  reactions  pro- 
duced by  the  separate  sections  of  the  coil  give  a  resultant, 
not  equal  to  their  algebraic  sum,  but  to  their  vector  sum, 
which  is  less  than  their  algebraic  sum. 

Since  armature  reaction  produces  a  distortion  of  the 
field,  a  curve  of  E.M.F.,  that  may  be  a  sine  curve  at  no 
load,  will  often  depart  widely  from  this  form  when  the 
generator  is  loaded.  The  distortion  of  the  wave-shape 
in  unitooth  machines  may  be  overcome  in  great  part  by 
careful  shaping  of  the  pole  pieces. 

While  the  armature  reaction,  due  to  a  lagging  current, 
lowers  the  terminal  E.M.F.  of  a  generator,  a  leading  cur- 
rent may  have  the  opposite  effect  by  adding  its  flux  to 
that  of  the  field. 

The  relation  between  the  E.M.F.  induced  in  the  sepa- 
rate armature  coils  and  that  delivered  at  the  terminals  of  a 
three-phase  machine  with  "Y"  connected  armature  is 
shown  in  Fig.  34.  Curves  A  and  B  represent  the  voltages 
measured  between  the  common  center  and  the  ends  of 
two  of  the  three  coils.  Curve  C,  formed  by  uniting  these 
"Y"  electromotive  forces,  gives  the  so-called  "delta" 
E.M.F.,  or  pressure  between  the  outer  terminals  of  the 
armature  coils,  and  therefore  the  measured  line  voltage. 
In  this  way,  if  the  line  voltage  is  found  to  be  1732  volts, 
the  voltage  of  any  of  the  three  coils  with  respect  to  the 

common  center  is  — j=-  =  1000. 

The  electromotive  force  induced  in  the  individual  arma- 
ture coils  of  a  standard,  "Y"  connected,  three-phase, 


OF   THE 

UNIVERSITY 


44      POLYPHASE  APPARATUS  AND  SYSTEMS. 

unitooth  machine   under  full  load   is   shown  in   Fig.   35. 
This  E.M.F.  is  termed  the  "  Y"  E.M.F.  of  the  machine. 

The  "delta"  E.M.F.,  or  curve  of  pressure  between  any 
two  of  the  machine  terminals  under  the  above  condition  of 
load,  is  shown  in  Fig.  36.  This  last  curve  can  be  readily 
obtained  by  adding  two  *-Y"  curves  for  any  particular 
condition  of  load  displaced  120  degrees. 


GENERATORS.  45 


CHAPTER   III. 
GENERATORS  (Concluded). 

Field  Excitation  and  Compounding.  —  The  voltage  of  a 
polyphase  generator  may  be  maintained  uniform,  under 
all  normal  conditions  of  balanced  load,  by  varying  the 
strength  of  the  field  excitation.  Where  the  load  is  unbal- 
anced the  voltage  of  the  more  heavily  loaded  phases  will 
be  lower  than  that  of  the  lightly  loaded  phases,  and  there 
are  no  means  of  equalizing  in  the  generator  an  unbalancing 
of  voltage  between  the  phases  due  to  unequal  load  condi- 
tions except  by  varying  the  number  of  armature  turns  in 
the  phase  to  be  adjusted,  or  by  other  equivalent  means. 
Such  means  of  equalizing  the  voltage  are  not  commer- 
cially employed  save  in  exceptional  instances,  because  any 
differences  of  voltage  are  more  simply  and  readily  adjusted 
by  means  of  separate  devices  known  as  potential  regu- 
lators, usually  inserted  in  the  feeder  circuit.  These  will 
be  found  described  in  another  part  of  this  book.  In 
speaking,  therefore,  of  compounding  a  polyphase  alter- 
nator we  mean  the  automatic  process  whereby  the  no  load 
balanced  E.M.F.  and  the  full  load  balanced  E.M.F.  are 
made  to  bear  any  desired  relation  to  each  other.  We 
speak  of  under-compound,  flat-compound,  or  over-com- 
pound, according  as  the  full  load  voltage  is  made  to  be  less 
than,  equal  to,  or  greater  than,  the  no  load  voltage. 

Compounding  devices  are  usually  arranged  to  hold  the 


46 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


voltage  constant  as  the  load  increases,  or  to  raise  the  vol- 
tage to  compensate  for  line  drop.  The  amount  of  com- 
pounding desirable  depends  on  the  conditions,  generators 
of  good  regulation  operating  under  favorable  conditions  as 
to  frequency  and  line  characteristics  requiring  less  than 
machines  of  poor  regulation  operating  over  a  line  where 
the  drop  is  high.  Moreover,  with  generators  of  small  or 

moderate  capacity 
compounding  ar- 
rangements are,  other 
things  being  equal, 
more  necessary  than 
with  very  large  ma- 
chines, for  the  reason 
that  in  the  latter  case 
the  systems  supplied 
by  large  generators 
are  so  extensive  that 
increases  of  load  in 
one  section  are  more 
or  less  balanced  by 
decreases  of  load  in 
another.  So  that,  at 
the  power  station,  the 
resultant  variation  of 


Fig.  37. 


load  on  the  dynamos  takes  place  slowly  and  can  readily  be 
compensated  for  by  hand  adjustment  of  the  field  rheostat. 

The  early  methods  of  compounding  alternators  followed 
the  analogy  indicated  by  the  continuous  current  generator, 
and  alternators  were  equipped  with  two  separate  field 
windings,  one  taking  the  practically  constant  excitation 
provided  by  the  exciter,  and  the  other  connected  in  series 


GENERATORS. 


47 


with  the  armature,  a  commutator  being  provided  to  rectify 
the  current.  To  pass  the  entire  armature  current  (duly 
rectified)  through  the  series  field  coils  was  frequently  found 
to  give  a  greater  than  necessary  series  excitation,  and  in 
such  cases  only  a  part  of  the  armature  current  was  taken, 
a  shunt  resistance  (located  within  the  armature  and  re- 


Auxiliary  Field, 

^MKXXIQQOOOO 


Fig.  38. 

volving  with  it)  being  provided  to  divert  any  necessary 
amount.  Another  shunt,  usually  adjustable,  external  to 
the  machine  and  connected  in  parallel  with  the  series 
field  winding,  still  further  reduced  the  amount  of  current 
passing  through  the  series  coils  and  permitted  the  amount 
of  compounding  to  be  variecj. 


48      POLYPHASE  APPARATUS  AND  SYSTEMS. 

This  method  of  compounding,  applied,  however,  chiefly 
to  revolving  armature  machines,  was  widely  employed, 
and  is  embodied  in  large  numbers  of  machines  which  are 
still  in  use. 

The  connections  of  a  three-phase  generator  with  com- 
pound field  windings  is  shown  in  Fig.  37.  A  three-part 
commutator  rectifies  the  currents  from  each  of  the  three- 
phase  circuits,  so  that  unbalancing  in  any  one  line  has  a 
minimum  effect  on  the  regulation.  The  rotating  shunt  is 
practically  the  common  center  of  the  coil,  giving  a  current 
in  the  series  field  due  to  about  i  per  cent  of  the  terminal 
voltage.  The  stationary  shunt  is  adjustable,  and  can  be 
varied  for  loads  of  different  power  factors.  It  also  serves 
to  prevent  sparking  at  the  commutator. 

In  another  polyphase  generator  (Fig.  38)  the  low  poten- 
tial current  for  the  series  field  is  derived  from  a  series 
transformer  within  the  armature.  All  phases  are  repre- 
sented in  the  primary.  The  compounding  field  current 
depends  upon  the  sum  of  the  currents  flowing  in  the  cir- 
cuits supplied  by  the  armature. 

The  demagnetizing  effect,  and  consequent  reduction  of 
voltage,  due  to  a  load  of  poor  power  factor,  has  been 
explained.  A  generator  so  loaded  requires  a  greater  field 
excitation  than  when  running  on  non-inductive  load.  The 
comparative  voltages,  with  loads  of  varying  power  factor 
and  the  same  excitation,  are  shown  in  Fig.  39.  Curve  a  is 
the  compounding  when  lights  are  the  chief  load,  and  b  the 
curve  when  the  load  consists  chiefly  of  motors.  It  will  be 
seen  that  a  generator  properly  over-compounded  for  a 
night  load  of  lamps  will  not  give  the  proper  voltage  for  a 


GENERATORS. 


49 


day  load  of  motors.  The  stationary  shunt  in  Fig.  37  will 
then  have  to  be  adjusted  for  the  varying  character  of  the 
load. 

The  method  of  compounding  above  described  has  the 
disadvantage  that  the  series  field  coils  usually  have 
the  same  difference  of  potential  above  ground  as  does  the 
armature  winding;  and  where  the  armature  is  wound  for 
a  high  potential  the  difficulty  of  insulation  and  the  danger 
to  life  from  accidental  contact  are  correspondingly  en- 


Kitowatts  Output 
Fig.  39. 

hanced.  This  method  has  the  further  important  disad- 
vantage that  it  does  not  automatically  compensate  for 
variations  in  power  factor,  since  with  a  given  value  of  the 
shunt  resistances  the  current  in  the  series  field  and  hence 
the  amount  of  compounding  depends  only  on  the  magni- 
tude of  the  current  flowing  in  the  armature,  regardless  of 
whether  this  is  lagging,  leading,  or  in  phase  with  the 
E.M.F.  For  all  modern  construction,  therefore,  this  ar- 
rangement has  been  virtually  abandoned,  particularly  in 
view  of  the  very  perfect  results  that  have  been  achieved 


50      POLYPHASE  APPARATUS  AND  SYSTEMS. 

by  methods  that  depend  for  their  operation  upon  control 
of  the  exciter  voltage,  a  method  that  will  be  fully  described 
under  the  caption  "Tirrill  Regulator,"  a  device  by  which 
any  degree  of  compounding  is  easily  secured  regardless  of 
power  factor  and  regardless^  too,  within  reasonable  limits, 
of  inequalities  of  speed. 

The  energy  required  to  excite  the  fields  of  good  com- 
mercial alternators  on  non-inductive  load  varies  from 
about  i  per  cent,  in  the  case  of  generators  of  500  kilowatts 
capacity  and  over,  to  2,  and  sometimes  3  per  cent  in 
smaller  machines.  The  energy  for  excitation  is  dependent 
also  on  the  number  of  poles,  for  the  reason  that  in  a 
machine  of  given  diameter  a  high  number  of  poles  means 
that  the  distance  between  poles  is  small  and  the  magnetic 
leakage  proportionately  high. 

The  exciting  dynamos  are  often  driven  from  a  pulley  on 
the  shaft  of  the  alternator.  The  exciters  are  also  often 
direct  connected  to  their  alternators,  and  where  the  alter- 
nator runs  at  high  speed  the  cost  of  the  direct-connected 
exciter  is  not  excessive.  In  the  case  of  very  slow  speed 
alternators,  the  cost  of  a  direct-connected  exciter  is  mate- 
rially higher.  Where  exciters  are  driven  either  by  belt 
connection  or  directly  from  their  alternators,  they  are 
subject  to  the  same  speed  variations  as  is  the  alternator. 
This,  in  turn,  causes  a  variation  in  the  exciter  voltage, 
and  the  effect  on  the  generator  voltage  is  accumulative, 
conditions  of  low  speed  occurring  simultaneously  with  low 
excitation  potential  so  that  the  effect  on  the  generator 
voltage  is  multiplied.  With  the  improved  compounding 
arrangements  now  obtainable  these  effects  are  minimized. 
Somewhat  better  operating  conditions  are  provided,  how- 
ever, where  the  exciters  are  actuated  from  separate  prime 


GENERATORS.  5 1 

movers,  steadier  excitation  potential  being  thereby  ob- 
tained. In  water  wheel  plants  this  arrangement  is  obtained 
by  equipping  the  exciters  with  independent  water  wheels. 
In  the  case  of  steam  plants  separate  engines  are  provided 
for  driving  the  exciters.  Such  engines  are,  however,  usu- 
ally of  small  capacity  and  realize  only  a  moderate  economy 
except  in  the  largest  stations,  so  that  many  steam  plants 
derive  their  excitation  from  motor  generator  exciters  in 
which  the  motor  receives  its  current  supply  from  the  alter- 
nating current  bus  bars.  With  this  arrangement  a  steam- 
driven  set  or  a  storage  battery  is  necessary  as  an  auxiliary 
to  provide  excitation  when  first  starting  up. 

The  use  of  motor-driven  exciters  is  to  some  extent  open 
to  the  same  objections  from  the  standpoint  of  voltage 
variation  as  exists  where  the  exciter  is  belted  or  direct 
connected  to  its  alternator.  The  speed  variations  of  a 
motor-generator  exciter  do  not,  however,  by  reason  of 
the  inertia  and  high  speed  of  its  revolving  parts,  have  the 
same  magnitude  as  those  of  the  main  generators;  and  the 
superior  economy  of  the  motor-generator  exciter  has  led 
to  its  wide  adoption  in  steam-driven  stations.  Considera- 
tions of  power  consumption  do  not  apply  in  hydraulic 
installations  to  the  same  extent  as  in  those  using  steam; 
nevertheless,  the  motor-generator  exciter,  by  reason  of  its 
simplicity  and  compactness,  is  frequently  used  in  water- 
power  stations.  In  such  stations  the  motor-generator  ex- 
citer has  in  some  cases  been  provided  with  a  direct-coupled 
water  wheel,  from  which  is  obtained  the  necessary  power 
at  the  beginning.  After  the  main  generators  have  been 
excited,  the  motor  is  switched  in  and  provides  the  power 
for  driving  the  exciter,  the  water-wheel  governor  thus 
practically  shutting  off  all  water  from  the  water  wheel. 


52      POLYPHASE  APPARATUS  AND  SYSTEMS. 

This  arrangement  provides  the  advantage  that  in  the  event 
of  stoppage  in  the  water  pipe,  the  entire  load  is  taken  by 
the  driving  motor,  and  vice  versa,  if  current  is  cut  off  the 
motor,  the  water  wheel  picks  up  and  the  operation  of  the 
set  continues  without  interruption.  This  arrangement, 
therefore,  provides  both  a  water  wheel  driven  and  a  motor- 
driven  exciter  set  without  the  use  of  two  exciter  genera- 
tors, and  insures,  moreover,  very  steady  speed  and  exciter 
potential  over  wide  ranges  of  load,  by  reason  of  the  com- 
bined action  of  the  water  wheel  and  motor,  which  divide 
the  loads  between  them  in  a  way  to  secure  constant  speed 
much  more  closely  than  could  be  secured  by  a  water 
governor  alone. 

Regulation.  —  Regulation,  sometimes  referred  to  as  in- 
herent regulation,  is  denned  in  four  or  five  different  ways; 
but  the  now  commonly  accepted  definition  is  the  percent- 
age rise  of  the  voltage  when  full  non-inductive  load  is 
thrown  off,  the  generator  speed  and  the  field  excitation 
remaining  constant.  From  what  has  been  said  in  connec- 
tion with  armature  windings,  it  follows  that,  as  a  rule, 
generators  with  unitooth  armatures  will  not  have  as  good 
a  regulation  as  the  multitooth  type.  However,  good  reg- 
ulation in  these  machines  can  be  obtained  at  a  slight  sac- 
rifice of  efficiency,  or  by  using  more  copper  in  the  con- 
struction of  the  generator,  and  thus  increasing  its  cost,  or 
by  the  use  of  a  high  magnetic  saturation  of  the  iron,  which 
increases  the  energy  required  for  excitation.  A  certain 
three-phase  unitooth  machine  of  large  output  gave  a  regu- 
lation of  61  per  cent,  from  full  load  to  10  per  cent  of  the 
load.  The  same  generator,  when  the  load  in  one  circuit 
was  reduced  50  per  cent,  did  not  rise  in  voltage  more  than 
5J  per  cent;  and  with  no  load  on  one  of  the  circuits,  the 


GENERATORS.  53 

others  being  fully  loaded,  the  greatest  variation  was  8  per 
cent.  The  standard  belt-driven  machines  of  the  unitooth 
construction  regulate  within  8  per  cent,  which  is  close 
enough  for  satisfactory  results  to  be  obtained,  even  without 
automatic  compounding.  Generators  of  the  multitooth 
construction  require  less  compounding.  The  standard  belt- 
driven  machines  of  this  type  have  a  regulation  of  6  per  cent 
or  thereabouts  when  designed  for  a  frequency  of  60  cycles. 

On  inductive  loads  the  regulation,  of  course,  is  not  so 
good.  The  generator  mentioned  above  as  having  a  non- 
inductive  regulation  of  6J  per  cent  will  require  approxi- 
mately 20  per  cent  more  ampere  turns  in  the  field  to  give 
full  load  voltage  when  it  is  supplying  current  to  motors  on 
a  circuit  where  the  power  factor  is  80  per  cent  lagging. 
The  regulation  under  these  conditions  is  about  16  per 
cent.  These  results  are  immensely  superior  to  those 
obtained  with  the  old  iron  clad  alternators,  which  often 
required  30  to  50  per  cent  increase  in  exciting  current  to 
maintain  constant  pressure  even  on  non-inductive  loads. 

A  construction  involving  poor  regulation  is  sometimes 
intentionally  used  in  generators  designed  for  special  pur^ 
poses,  for  instance,  in  alternating  arc  lighting,  where  a 
constant  current  is  required.  Generators  of  poor  regula- 
tion are  sometimes  used  also  in  certain  kinds  of  electric 
smelting  where  a  constant  energy  output  from  the  gener- 
ator is  required.  The  process  is  started  at  a  certain  vol- 
tage, and  as  the  resistance  of  the  external  circuit  decreases 
the  terminal  voltage  falls  in  ratio  to  the  increase  of  current. 

Efficiency  and  Losses.  —  Fig.  40  gives  the  efficiency 
curves  of  a  3000  kilowatt  three-phase  water  wheel-driven 
generator,  and  shows  the  individual  losses  in  the  machine. 
It  will  be  noted  that  the  highest  efficiency  is  reached  at 


54      POLYPHASE  APPARATUS  AND  SYSTEMS. 

full  load,  and  does  not  diminish  up  to  25  per  cent  overload, 
the  losses  at  full  load  being  only  about  2.7  per  cent  of  the 
total  output.  The  efficiency  at  half  load,  95.5  per  cent,  is 
most  excellent.  The  loss  due  to  bearing  friction  and  to 
windage  is  constant  at  about  two-thirds  of  i  per  cent 
for  all  loads.  The  PR  loss  in  the  field  varies  little  from 
no  load  to  full  load,  showing  that  the  generator  is  easy  to 
regulate.  The  core  loss  is  practically  constant,  varying  only 
from  38  kilowatts  at  no  load  to  38.5  kilowatts  at  full  load. 
Other  things  being  equal  generators  for  engine  connection 
will  have  an  apparently  higher  efficiency  than  those  other- 
wise driven,  especially  at  light  loads,  as  the  friction  losses 
are,  as  a  rule,  reduced  by  the  omission  of  all  bearing 
losses,  these  being  considered  as  chargeable  to  the  engine 
losses. 

The  efficiencies  of  generators,  as  usually  given,  do  not 
include  the  losses  in  the  exciter.  As  the  exciter  efficiency 
is  from  80  to  90  per  cent,  and  the  field  loss  about  i  per 
cent,  the  reduction  of  the  generator  efficiency  due  to  this 
source  will  seldom  be  greater  than  0.2  per  cent. 

Power  Factor.  —  Manufacturers'  statements  as  to  regu- 
lation and  temperature  of  generators  are  usually  given  on 
the  basis  of  100  per  cent  power  factor,  i.e.,  100  per  cent 
energy  load.  Generators  are  constructed  with  reference 
to  operation  at  the  full  rated  current  and  voltage,  i.e., 
voltampere  output,  at  power  factors  as  low  as  80  per  cent 
(80  per  cent  energy  load).  When  operating  at  lower  than 
100  per  cent  power  factor,  the  regulation  is  not  as  good. 
The  regulation  of  standard  alternators  for  rated  voltam- 
pere output,  80  per  cent  power  factor,  is  approximately: 

25%  in  cases  where  it  is  stated  as  10%  at  100%  power  factor. 
22%  in  cases  where  it  is  stated  as  8%  at  100%  power  factor. 
1 8%  in  cases  where  it  is  stated  as  6%  at  100%  power  factor. 


GENERATORS. 


55 


JO    GO    7O     SO    SO    /OO  //O   /2O  /3O 


Fig.  40. 


56      POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  commercial  efficiency  of  the  average  generator  is 
about  i  per  cent  less  for  the  80  per  cent  power-factor  load 
than  for  the  100  per  cent  power- factor  load,  both  rated  in 
voltamperes. 

The  result  of  operating  at  lower  power  factor  is  to 
increase  the  heating  of  the  field  coils  due  to  the  greater 
excitation  required.  This  increase  at  80  per  cent  power 
factor  is  about  5  degrees  C.  The  temperatures  as  given 
for  100  per  cent  power  factor  are  applicable  for  80  per 
cent  power  factor  to  all  parts  of  the  several  machines 
except  to  the  field  coils  as  noted. 

Speed.  —  In  nearly  all  classes  of  moving  machinery  - 
engines,  water  wheels,  pumps,  dynamos,  motors,  etc.  - 
the  weight  and  cost  are  lower  when  the  apparatus  is 
designed  for  high  speed  than  when  intended  to  run  at 
a  low  speed.  Generally  the  weight  and  cost  go  down 
as  the  speed  goes  up.  Limitations  to  the  saving  attainable 
by  increase  of  speed  are  found  in  the  extreme  mechanical 
stresses  which  are  introduced  at  the  higher  speeds,  these 
stresses  imposing  on  the  designer  the  use  of  costly  mate- 
rials or  heavy  sections  in  order  to  obtain  the  necessary 
strength  of  parts,  or  obliging  him  at  increased  cost  to 
modify  the  electrical  design  in  order  to  favor  the  mechan- 
kal.  There  is,  therefore,  for  a  machine  of  given  capacity 
and  characteristics  a  certain  speed  corresponding  to  mini- 
mum cost.  It  is  not  possible,  except  in  special  cases,  to 
build  generators  commercially  for  this  theoretically  desir- 
able speed,  because  the  minimum  cost  speed  for  the  gen- 
erator may  not  coincide  with  the  minimum  cost  speed  of 
the  prime  mover.  Moreover,  conditions  of  operation  may, 
and  frequently  do,  demand  the  selection  of  a  speed  that  is 
far  from  corresponding  to  minimum  first  cost.  The  choice 


GENERATORS.  57 

of  speed,  therefore,  is  fixed  by  a  number  of  considerations 
in  which  the  question  of  cost  is  only  one.  In  alternating 
current  generators  and  motors,  furthermore,  there  exists  an 
additional  limitation  in  respect  to  choice  of  speed,  namely, 
that  imposed  by  frequency.  Since  frequency  is  a  func- 
tion both  of  speed  and  of  number  of  poles,  it  follows  that 
for  a  given  periodicity  the  number  of  speeds  available  is 
confined  to  such  values  as  will  conform  to  the  equation, 

120  X  Frequency 
Speed  =          j —  •    Thus  for  a  60  cycle  generator 

the  highest  possible  speed  is  3600  revolutions  per  minute, 
corresponding  to  a  bipolar  machine.  The  next  lower 
speed  is  that  corresponding  to  a  four-pole  generator, 
namely,  1800  revolutions  per  minute.  For  25  cycles  under 
similar  conditions  the  speeds  would  be  1500  and  750  revo- 
lutions respectively. 

In  the  older  machines  operating  at  125  and  133  cycles, 
speeds  of  1500  to  2000  revolutions  were  common.  Such 
high  speeds  are  disadvantageous  in  belt-driven  designs, 
especially  in  the  larger  units,  and  to  secure  low  speeds  at 
these  high  frequencies  required  a  design  having  a  large 
number  of  poles,  which  in  turn  results  in  an  expensive 
and  inefficient  machine.  The  advisability  of  more  moder- 
ate frequencies  was  therefore  indicated  as  being  more 
suitable,  both  from  the  generator  standpoint  and  from 
considerations  of  lower  inductive  loss  in  the  external  cir- 
cuits. Various  frequencies  of  relatively  low  magnitude 
have  become  standardized,  each  chosen  with  respect  to 
the  conditions  to  be  met,  and  frequencies  above  60  cycles 
are  now  rare  save  in  the  older  plants.  The  majority  of 
polyphase  belt-driven  generators  in  actual  use  in  this 
country  are  wound  for  60  cycles,  while  on  the  Continent 


58      POLYPHASE  APPARATUS  AND  SYSTEMS. 

and  in  England  the  prevailing  frequency  is  50  cycles. 
The  standard  belt-driven  generators  constructed  by  one  of 
the  largest  manufacturers  have  the  following  number  of 
poles  and  speeds  for  the  respective  outputs  when  designed 
for  60  cycles: 

K.W.  POLES  R.P.M. 

50  6  1200 

75  8  900 

100  8  900 

150  12  600 

200  12  600 

300  i 6  450 

45O  20  360 

650  20  360 

75°  24  3°° 

The  alternating-current  generator  is  far  from  being 
like  a  direct-current  machine  —  a  flexible  piece  of  appa- 
ratus in  respect  to  speed.  The  speed  cannot  be  altered 
more  than  about  10  per  cent  either  way  from  that  for 
which  it  is  designed,  without  appreciably  affecting  the 
constants  of  the  generator  and  of  the  apparatus  to  which 
the  generator  is  supplying  current. 

Parallel  Running.  —  In  modern  alternating-current 
plants  parallel  operation  is  necessary  in  order  to  effect  a 
reduction  in  the  number  of  circuits  and  transmission 
lines.  Other  advantages  are  economy,  simplicity,  and  re- 
liability of  operation.  Polyphase  generators,  as  now  de- 
signed, can  be  operated  in  multiple  without  any  difficulty. 

The  principal  requirement  in  the  generators  is  that  they 
shall  have  a  moderate  armature  impedance  and  a  uniform 
air  gap.  Too  small  an  impedance  permits  an  excessive 
exchange  of  current  with  slight  inequality  of  the  field 
excitation  of  the  machine,  and  a  dangerous  flow  if  the 
generators  are  connected  up  when  they  are  not  quite  in 


GENERATORS.  59 

step.  Generators  having  a  large  armature  impedance 
will  operate  in  parallel;  but,  owing  to  the  small  synchron- 
izing current  that  can  be  exchanged,  the  condition  is  not 
stable,  and  the  generators  are  liable  to  lead  alternately  in 
speed  or  "  hunt." 

When  generators  are  run  in  parallel  the  field  excitation 
must  be  adjusted  on  each  to  that  value  which  would  give 
the  same  voltage  if  the  machine  were  operating  alone.  If 
this  is  not  done,  lagging  currents  will  be  taken  by  the 
armature  of  that  machine  whose  field  is  too  strongly  ex- 
cited, these  lagging  currents  acting  to  correct  the  over- 
magnetization  by  opposing  the  flux  which  the  field  coils 
produce.  Conversely,  those  machines  that  are  under- 
excited  will  take  leading  current.  The  result  is  that  idle 
currents,  possibly  of  large  magnitude,  circulate  between 
the  machines  and  through  their  armature  windings,  over- 
heating the  conductors  or  reducing  the  effective  output 
which  for  the  same  temperature  rise  could  otherwise  be 
obtained.  With  proper  adjustment  of  excitation  these 
idle  currents  disappear.  When  this  condition  is  attained 
the  sum  of  the  readings  of  the  ampere-meters  of  the  sev- 
eral generators  will  be  equal  to  the  total  amperes  delivered 
to  the  external  circuits.  The  same  condition  is  indicated 
when  the  generators  are  found,  by  means  of  suitable  in- 
struments, to  be  delivering  their  respective  outputs  at  the 
same  power  factor,  taking  into  account  the  fact  that  the 
power  factor  of  all  generators  will  have  the  same  sign,  i.e., 
all  generators  delivering  lagging  current,  or  all  delivering 
leading  current,  or  all  working  at  unity  power  factor, 
according  to  the  character  of  the  load. 

The  requirements  for  the  prime  mover  are  uniform  speed 
and  uniform  angular  rotation.  In  belt-driven  generators 


60      POLYPHASE  APPARATUS  AND  SYSTEMS. 

the  pulleys  must  all  have  the  same  dimensions.  The  belts 
must  be  watched  to  see  that  they  do  not  slip.  These  two 
points  must  be  especially  observed  in  generators  driven 
from  the  same  shaft.  The  speed  regulation  of  engines 
operating  direct-connected  alternators  in  parallel  is  dis- 
cussed in  the  following  section.  Water  wheels  and  steam 
turbines  have  an  absolutely  uniform  angular  rotation,  and 
are  the  best  prime  movers  for  parallel  running. 

Synchronism  of  two  polyphase  generators  is  determined 
by  some  form  of  phase  indicator.  A  common  arrange- 
ment consists  of  two  transformers,  the  primaries  of  which 
are  connected  to  each  generator,  care  being  taken  that 
the  connections  are  made  to  similar  phases.  The  second- 
aries are  connected  in  series  with  one  or  two  lamps  in  cir- 
cuit. With  the  transformer  secondaries  connected  in  a 
certain  relation,  the  machines  are  in  synchronism  when 
the  lamps  cease  to  glow.  If  the  secondary  of  one  trans- 
former is  connected  in  the  opposite  sense,  synchronism  is 
indicated  when  the  lamps  are  at  their  max'mum  brilliancy. 
This  latter  connection  is  preferable  because  the  instant  of 
maximum  brilliancy  is  susceptible  of  fairly  accurate  deter- 
mination, whereas  with  the  transformers  connected  for 
synchronizing  "dark"  the  operator  must  rely  on  his  judg- 
ment to  determine  the  instant  of  time  lying  midway  between 
the  moment  when  the  lamps  cease  to  glow  and  when  they 
begin  to  glow  again.  As  soon  as  synchronism  is  indicated 
by  the  lamps,  according  to  one  or  the  other  of  the  two 
above  methods,  the  machines  may  then  be  thrown  in 
parallel  by  the  main  switches.  Where  alternators  are 
equipped  with  series  fields,  the  commutators  must  be  con- 
nected by  an  equalizer  to  place  the  series  windings  in  mul- 
tiple. The  connections  and  station  instruments  required 


GENERATORS. 


6l 


for  the  process  of  throwing  generators  in  parallel  and  oper- 
ating them  continuously,  as  used  extensively  in  this  country, 


Connect/ona  for 
theCriff/sie  Governor  Contro/  A/ott 
am?  Sw/tcti  wtien 


Fig.  41. 

are  shown  in  Fig.  41.  This  figure  shows  the  type  of  gen- 
erator more  commonly  built  at  the  present  time,  that  is, 
one  having  separately  excited  field  winding  only. 


62 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


It  does  not  follow  that  because  one  phase  of  a  poly- 
phase circuit  is  synchronized,  the  other  phases  are  ready 
for  parallel  connection.  It  is  necessary  that  when  a  num- 
ber of  machines  are  first  installed  for  operating  in  parallel, 
the  connections  should  be  such  as  to  give  the  same  phase 
rotation  in  all  the  machines.  The  circuits  can  be  tested 
out,  for  proper  connection,  by  means  of  two  sets  of  phase 
lamps.  - 

In  the  diagram    (Fig.   42)   temporary  transformers  are 


fo  Bus  Bars, 


Synchr  oni  zing 
Lamps. 


"wvww 


jVWWV i 

L\A/W\A ? 

,\ 

I 


*Tb  Generator: 
Fig.  42. 


shown  connected  to  a  different  phase  of  the  circuit  from 
that  in  which  are  the  permanent  lamps.  Connection 
should  first  be  made  with  the  outside  blades,  as  shown  by 
the  dotted  lines,  to  prove  that  the  two  sets  of  lamps  become 
alight  and  are  extinguished  at  the  same  time.  By  the 
separate  connections  of  the  temporary  transformers,  it  can 
be  ascertained  if  the  machines  are  properly  connected  to 


GENERATORS.  63 

the  synchronizing  switches.  The  connections  are  correct 
when  both  lamps  are  simultaneously  dark  or  simultane- 
ously bright,  according  to  whether  the  secondaries  of  the 
transformers  are  connected  in  the  manner  shown  by  the 
figure,  or  whether  in  connecting  them  in  series  one  of 
the  secondaries  is  connected  in  the  reverse  sense. 

With  the  continual  increase  in  the  size  of  units  it  was 
appreciated  that  more  perfect  means  of  synchronizing 
were  desirable  than  those  provided  by  the  lamp  arrange- 
ments just  described,  for  the  reason  that  more  serious 
consequences  ensue  if  large  generators  having  heavy  re- 
volving parts  are  switched  in  out  of  phase.  Such  a 
device  is  provided  in  the  synchronizer  or  " synchroscope." 
This  device  consists  essentially  of  a  small  single-phase 
induction  motor,  of  which  the  stator  is  excited  from  the 
station  bus  bars  and  the  rotor  excited  by  split-phase 
currents  from  the  alternator  which  is  about  to  be 
synchronized.  A  revolving  field  is  thus  produced  in 
the  synchronizer  and  the  rotor  will  revolve  at  a  speed 
which  is  governed  by  the  difference  between  the  frequency 
impressed  upon  the  stator  and  that  impressed  upon  the 
rotor.  To  the  rotor  shaft  of  the  synchronizer  is  attached 
a  pointer,  which,  at  first  revolving  rapidly,  will  revolve 
more  and  more  slowly  as  the  frequency  delivered  to  the 
rotor  approaches  that  delivered  to  the  stator.  When  the 
frequency  is  the  same  in  each  member  the  velocity  of  the 
rotating  field  will  be  zero  and  the  pointer  will  be  station- 
ary. This  condition  indicates  equality  of  frequency,  but 
not  necessarily  correctness  of  phase.  For  the  latter  condi- 
tion there  is  but  one  relative  position  of  stator  and  rotor  in 
which  the  flux  generated  by  stator  and  rotor  is  simultane- 
ously in  the  same  direction;  hence,  while  equality  of  fre- 


64      POLYPHASE  APPARATUS  AND  SYSTEMS. 

quency  is  indicated  by  a  stationary  position  of  the  pointer 
at  any  angle,  the  machines  are  in  phase  only  when  the 
pointer  is  stationary  in  a  predetermined  position  (usually 
pointing  vertically  upward).  In  other  positions  the  angle 
occupied  by  the  pointer  indicates  the  instantaneous  dif- 
ference of  phase  between  the  machines. 

From  what  has  preceded  it  will  be  apparent  that  a 
meritorious  feature  of  this  device  is  its  ability  to  indicate 
whether  the  generator  which  is  being  synchronized  is 

running  too  fast  or  too  slow, 
because  if  running  too  fast  the 
frequency  impressed  upon  the 
rotor  will  be  greater  than  that 
impressed  upon  the  stator  and 
a  progressive  forward  rotation 
of  the  pointer  will  result.  Con- 
versely, if  the  generator  being 
synchronized  is  running  under 
speed,  the  pointer  will  have  a 

retrograde  motion.  Synchronizers  of  this  type  (which  have 
the  general  appearance  shown  in  Fig.  43)  provide  the 
means  of  determining  correctness  of  phase  with  the  utmost 
nicety,  and  are  widely  used  where  accurate  synchronizing 
is  desired. 

Speed  Regulation  of  Engines.  —  Steam  engines  intended 
for  direct  connection  to  alternators  which  supply  current 
to  rotary  converters  or  to  synchronous  motors,  or  which 
are  operated  in  parallel,  should  be  designed  to  have  an 
angular  rotation  as  nearly  uniform  as  possible.  Other- 
wise the  oscillations  in  the  relative  motions  of  the  genera- 
tors or  of  the  generators  and  synchronous  apparatus  may 
produce  an  excessive  exchange  of  currents  —  a  state  of 


GENERATORS.  65 

affairs  known  as  " hunting,"  " pulsation,"  " pumping,"  or 
"surging." 

The  amount  of  deviation  from  the  position  of  abso- 
lutely uniform  angular  speed  permissible  for  satisfactory 
work  depends  upon  a  number  of  conditions.  It  is  affected 
by  the  design  of  the  generator,  and  rather  more  by  the 
difficulties  of  operating  synchronous  apparatus.  For  the 
majority  of  cases  it  is  customary  to  specify  an  allowable 
angular  variation  of  about  2\  degrees  of  phase  from  the 
mean.  This  means  that  in  engines  direct  connected  to 
alternators  of  2  n  poles  the  position  of  each  revolving 

2^  degrees  .  f 

part  should  not  differ  more  than  — ^—          -  in  circumfer- 

n 

ence  from  the  position  it  would  have  at  absolutely  uniform 
rotation.  Thus,  in  a  40-pole  alternator  the  maximum 
allowable  deviation  from  the  position  of  uniform  rotation 

2^ 

would  be  —  or  J-  degree  of  circumference. 
20 

The  above  expresses  the  regulation  of  the  engine  as  a 
deviation  in  position  from  that  of  absolutely  uniform  rota- 
tion in  degrees  of  total  circumference  measured,  for  ex- 
ample, on  the  circumference  of  the  fly  wheel. 

Single  cylinder  or  tandem  compound  engines  cannot,  as 
a  rule,  give  as  good  results  as  engines  whose  cranks  are 
quartering. 

The  difficulty  of  parallel  operation  of  alternators  is  not, 
however,  very  often  due  to  the  cyclic  irregularity  of  the 
turning  moment  of  the  prime  mover  (which,  by  changing 
the  frequency  during  each  revolution,  may  cause  hunting 
of  synchronous  apparatus).  Nor  has  the  design  of  the 
alternators  any  material  influence  upon  their  successful 
parallel  running.  It  was  demonstrated  some  years  ago, 


66      POLYPHASE  APPARATUS  AND  SYSTEMS. 

under  conditions  where  all  the  requirements  of  uniform 
angular  velocity  had  apparently  been  complied  with  and 
it  was  still  impossible  to  keep  the  generators  satisfactorily 
in  step,  that  a  condition  of  perfect  stability  in  parallel 
operation  was  secured  by  applying  anti-hunting  governors 
to  the  prime  movers.  Where  the  engine  governor  acts 
very  freely  the  impulses  due  to  changes  of  load  are  empha- 
sized and  the  governors  tend  to  overrun  or  hunt.  The 
cure  is  in  the  use  of  a  dashpot  device  which  will  retard  for 
a  brief  period  any  motion  of  the  valve  mechanism,  but 
which  will  yield  to  continued  pressure,  thereby  preventing 
sudden  jumps  in  speed  while  still  allowing  the  governor 
properly  to  control  the  steam  admission  with  variations  of 
load. 

Another,  although  rare,  condition  liable  to  prevent  sat- 
isfactory parallel  operation  is  sometimes  found  where  a 
generator  of  exceptionally  close  regulation  is  connected  to 
an  engine  whose  revolving  parts,  including  the  rotating 
member  of  the  alternator,  are  of  such  weight  and  operate 
at  such  a  speed  of  rotation  as  causes  the  number  of  engine 
impulses  to  coincide  with  the  natural  period  of  oscillatory 
vibration  of  the  entire  reciprocating  and  revolving  parts 
considered  as  a  torsion  pendulum  swinging  on  either  side 
of  the  point  of  synchronism.  This  fact  becomes  impor- 
tant with  generators  of  close  regulation,  owing  to  the  large 
value  of  synchronizing  current  which  passes  when  the 
machines  get  out  of  step,  and  the  tendency  of  this  large 
current  is  not  only  to  pull  the  machines  back  into  syn- 
chronism, but  to  cause  the  lagging  machine  to  overrun 
and  become  leading  with  respect  to  the  other.  In  doubt- 
ful cases  careful  calculation  will  indicate  the  probability, 
or  otherwise,  of  this  action,  which  can  be  guarded  against 


Of    THE 

UNIVERSITY 


GENERATORS.  67 

by  a  change  or  rearrangement  of  the  weights  so  as  to  alter 
the  so-called  natural  period  of  vibration. 

Methods  of  Driving  Generators.  —  The  method  of  driv- 
ing a  generator  from  its  prime  mover  is  determined  mainly 
by  the  size  of  the  generator  and  the  type  and  speed  of  the 
prime  mover.  Polyphase  units  up  to  200  kilowatts  are 
usually  belted,  unless  the  prime  mover  consists  of  a  water 
wheel  of  high  speed,  or  special  conditions  favor  direct 
connection  to  an  engine.  The  mechanical  arrangement  of 
a  small  belted  generator  is  shown  by  Fig.  44.  The  yoke 
rests  on,  and  is  sometimes  an  integral  part  of,  the  bedplate, 
which  also  supports  two  bearings.  The  pulley  is  over- 
hung. 

The  method  of  belt-driving  larger  units  is  shown  in 
Fig.  45.  The  bedplate  is  extended,  and  carries  a  third  or 
outboard  bearing  which  partly  relieves  the  inner  bearing 
of  the  belt  strain  and  the  weight  of  the  pulley. 

Generators  designed  for  water-wheel  connection  are 
usually  provided  with  bedplate,  shaft,  and  two  bearings. 
These  machines  are  self-contained  for  the  more  perfect 
alignment  of  the  bearings.  Fig.  46  illustrates  the  general 
arrangement  of  generators  of  500  kilowatts  capacity  and 
above.  A  half-coupling  is  provided,  which  is  machined 
to  a  close  fit  with  the  other  half  furnished  with  the  water 
wheel. 

Generators  for  direct  connections  to  engines  are  built 
without  bedplate,  shaft,  or  bearings.  The  yoke  rests  on  a 
thin  iron  soleplate  supported  by  a  suitable  foundation. 
The  engine  bearing  serves  also  for  the  inner  bearing  of  the 
generator.  The  outboard  bearing  rests  on  a  separate  cap. 
It  is  usually  furnished  with  the  engine,  and  is  of  a  design 
uniform  with  the  inner  bearing.  The  engine  shaft  ex- 


68  POLYPHASE   APPARATUS   AND   SYSTEMS. 


GENERATORS. 


69 


f/ooefs-  TuT/iowjj  jof  uq<jrf6aiJi  H-Vf-J/  jo  u»j 


70      POLYPHASE  APPARATUS  AND  SYSTEMS. 

tended  carries  the  revolving  element  of  the  electrical  unit 
(Fig.  47)- 

Polyphase  generators  above  500  kilowatts  should  pref- 
erably be  direct  coupled  to  the  prime  mover.  The  method 
of  driving  large  generators  by  belts  or  ropes  necessitates  a 
large  extension  of  the  base  and  a  heavy  pulley,  and  is 
mechanically  awkward.  This  method  of  driving  may  be 
used  in  exceptional  cases,  as,  for  instance,  in  connection 
with  a  wheel  plant  already  installed,  operating  under  a 
very  low  head  at  a  low  speed.  The  increased  cost  of 
extended  shaft,  outboard  bearing,  and  pulley  will,  how- 
ever, go  far  towards  offsetting  the  increased  cost  of  a 
slower  speed  generator,  for  direct  connection,  which  does 
not  require  these  parts. 

Polyphase  generators  are  direct  connected  to  water 
wheels  either  by  a  vertical  or  by  a  horizontal  shaft.  While 
most  generators  in  this  country  run  from  horizontal  shaft 
turbines,  the  advantages  of  the  vertical  shaft  construc- 
tion, particularly  for  large  units,  are  causing  a  wider  adop- 
tion of  this  form.  These  advantages  lie  in  the  saving  of 
floor  space,  which  means  a  smaller  power  house,  and  in 
more  responsive  wheel  regulation.  The  shaft  is  out  of 
sight,  the  stresses  on  the  shaft  are  practically  those  due  to 
torsion  only,  which  permits  a  smaller  shaft  to  be  used,  and 
the  appearance  of  the  unit  is  as  a  whole  most  pleasing. 
The  principal  disadvantages  operating  to  prevent  a  wider 
use  of  this  type  were,  first,  an  anticipated  difficulty  (which 
has  been  overcome)  of  properly  supporting  the  weight  of 
the  shaft  with  its  revolving  parts,  and,  second,  the  fact 
that  the  vertical  type  is  not  quite  so  accessible  for  inspec- 
tion and  repair.  The  advantages  named  have,  however, 
caused  the  adoption  of  this  type  in  many  important  installa- 


GENERATORS.  71 

tions,  among  which  may  be  mentioned  several  of  the  power 
stations  at  Niagara  Falls,  the  large  hydraulic  station  of 
the  Mexican  Light  and  Power  Company  at  Necaxa,  and 
the  plant  of  the  Great  Northern  Power  Company  at 


Fig.  48. 


Duluth,  Minnesota,  which  has  been  laid  out  for  an  ulti- 
mate capacity  of  eight  machines,  each  of  7500  kilowatts. 

Fig.  48  shows  the  typical  construction  of  most  large 
vertical  shaft  generators.  The  machine  illustrated  is  of 
the  revolving  field  type,  eight  poles,  7500  kilowatts,  375 
revolutions  per  minute,  6600  volts,  25  cycles. 


72      POLYPHASE  APPARATUS  AND  SYSTEMS. 

In  the  horizontal  type  sometimes  a  shaft  extension  and 
outboard  bearing  is  used,  the  water  wheel,  properly  housed, 
occupying  the  space  between  the  inboard  and  outboard 
bearings.  Such  an  arrangement  is  peculiarly  adapted  for 
use  with  impact  wheels  and  provides  an  arrangement 
whereby  perfect  and  permanent  alignment  of  bearings  is 
secured.  Machines  of  this  construction  are  used  in  the 
power  plants  of  the  Big  Cotton  wood  Electric  Company; 
the  Pioneer  Electric  Company,  Ogden,  Utah;  and  the 
Southern  California  Power  Company,  Redlands,  Cal. 

More  often  the  generator  is  equipped  with  but  two  bear- 
ings and  is  driven  by  the  water  wheel  through  a  coupling. 
This  construction  is  followed  in  the  3000  kilowatt  genera- 
tor depicted  in  Fig.  26. 

Where  engines  are  direct  connected  to  polyphase  gen- 
erators it  is  customary  for  the  electrical  manufacturers  to 
furnish  the  machine  without  shaft,  base,  or  bearings.  For 
the  same  speed,  therefore,  engine-driven  generators  are 
cheaper  than  those  driven  by  water  wheels.  It  must  not 
be  forgotten,  however,  that  engine  speeds  are  limited  by  a 
number  of  conditions,  while  water  wheels  are  practically 
limited  in  speed  only  by  the  head  obtainable. 

Fig.  49  illustrates  a  three-phase  generator  of  1200  H.P. 
capacity,  direct  connected  to  an  engine  running  at  94  revo- 
lutions per  minute. 

This  generator  is  direct  coupled  to  a  Corliss  type  of 
engine  of  1300  indicated  horse  power,  running  at  94  revo- 
lutions. It  has  32  poles,  and  gives  a  current  at  5000  volts 
and  a  frequency  of  25  cycles.  The  armature  windings 
consist  of  96  coils,  three  for  each  pole,  or  two  slots  per 
phase  per  pole.  The  windings  are  Y  connected.  The 
field  coils  are  flat  strip  copper,  i  inch  by  A  inch,  wound  on 


GENERATORS. 


edge,  and  insulated  by  intervening  layers  of  paper.  As 
the  exciting  current  has  a  pressure  of  not  greater  than 
120  volts,  the  potential  at  the  terminals  of  each  field  spool 
is  about  four  volts.  The  efficiency  of  the  generator  is 


Fig.  49. 

95  J  per  cent  at  full  load,  94 J  per  cent  at  three-quarter 
load,  92^  per  cent  at  half  load,  and  87  per  cent  at  quarter 
load.  The  regulation  on  non-inductive  load  is  6  per 
cent,  and  the  exciting  current  about  120  amperes. 

Engine-driven  generators  are  sometimes  constructed  with 
their  field  magnets  built  out  as  integral  parts  of  the  engine 


74      POLYPHASE  APPARATUS  AND  SYSTEMS. 

fly  wheel,  they  with  their  windings  being  fastened  to  the 
external  periphery.  These  machines  are  known  as  Fly- 
wheel Alternators.  The  largest  machines  of  this  type  yet 
constructed  were  designed  by  the  Westinghouse  Company 
for  the  Manhattan  Railway  Company  of  New  York. 
They  have  a  nominal  rating  of  5000  kilowatts  with  50  per 
cent  overload  capacity,  height  42  feet,  diameter  of  revolv- 
ing part  about  32  feet,  weight  185  tons.  The  revolving 
field  construction  consists  of  a  steel  hub  which  supports  a 
dovetailed  structure  of  steel  web  plates  which  form  the 
driving  spider.  The  field  poles  and  rim  are  built  up  with 
overlapped  plates  of  thin  sheet  steel,  the  length  of  each 
plate  being  equal  to  two  poles.  The  plates  are  dovetailed 
into  the  driving  spider,  and  the  rim  and  poles  with  their  steel 
end  plates  are  separately  bolted  together.  In  the  body  of 
the  field  poles,  at  intervals  of  about  3  inches,  ventilated 
spaces  or  ducts  are  provided.  These  spaces  extend  in- 
ward to  the  laminated  steel  rim  of  the  fly  wheel  to  large 
openings  in  the  cast-iron  driving  rim.  The  ventilating 
ducts  in  the  revolving  field  register  with  corresponding 
ducts  in  the  external  stationary  armature.  The  field-pole 
tips  are  beveled  at  the  edges  to  produce  a  magnetic  dis- 
tribution of  such  a  form  that  with  a  star  type  of  three- 
phase  winding  the  E.M.F.  wave  will  be  practically  a  sine 
wave  at  no  load.  The  speed  is  75  revolutions  per  minute, 
poles  40,  frequency  25.  The  field  requires  225  amperes  at 
200  volts  when  the  machine  is  delivering  its  rated  current 
at  11,000  volts  on  a  non-inductive  load.  About  15  per 
cent  more  current  is  required  when  the  armature  delivers 
its  full  rated  output  at  normal  voltage  to  a  circuit  having  a 
power  factor  of  90  per  cent.  The  regulation  is  such  that 
if  load  of  263  amperes  per  phase,  11,000  volts,  and  with 


GENERATORS.  75 

100  per  cent  power  factor  be  thrown  off,  the  potential  will 
rise  not  more  than  6  per  cent;  field  excitation  and  speed 
remaining  constant.  It  is  calculated  on  non-inductive 
load,  that  the  efficiency  will  range  from  90  per  cent  at 
quarter  load  to  96^  per  cent  at  full  load,  mechanical  fric- 
tion not  being  included. 

Polyphase  generators,  designed  for  driving  by  steam  tur- 
bines, present  certain  special  features  of  construction  by 
reason  of  the  high  rotative  speeds  at  which  they  operate. 
The  same  features  of  construction  are  to  a  certain  extent 
used  in  the  design  of  generators  for  direct  connection  to 
extra  high  speed  water  wheels.  The  distinguishing  char- 
acteristic of  these  extra  high  speed  machines  is  the  ratio 
of  length  to  diameter,  which  in  some  cases  may  be  equal 
to  unity  or  even  greater,  whereas  in  machines  for  more 
moderate  speeds  the  length  is  almost  invariably  but  a 
small  fraction  of  the  diameter.  The  restriction  of  diam- 
eter follows  from  the  necessity  of  keeping  the  peripheral 
speed  within  practicable  limits.  Even  with  the  small 
diameters  chosen  the  peripheral  velocity  reaches  in  some 
designs  as  high  as  12,000  feet  per  minute.  Even  at  these 
speeds  the  diameters,  relatively  speaking,  are  so  small 
that  a  considerable  length  is  required  in  the  armature  in 
order  to  provide  the  requisite  quantity  of  iron.  Machines 
of  smaller  weight  for  the  same  capacity  would  result  could 
the  armature  diameter  be  increased,  but  such  a  course  is 
prohibited  by  considerations  of  peripheral  velocity. 

The  high  rotative  speeds  imposed  from  considerations  of 
turbine  design,  ranging  from  about  3000  to  1800  revolutions 
in  machines  of  500  kilowatt  capacity,  to  about  750  revolu- 
tions per  minute  in  the  largest  sizes,  require,  for  all  of  the 
commercial  frequencies,  a  small  number  of  poles.  Even 


76      POLYPHASE  APPARATUS  AND  SYSTEMS. 

at  the  lowest  speeds  turbine  generators  for  25  cycles  usu- 
ally have  but  four  poles.  The  values  chosen  for  the 
densities  in  the  copper  and  iron  circuits  are  not  substan- 
tially different  from  those  appropriate  to  slower  speed 


Fig.  50. 

designs,  although  by  the  nature  of  the  case  the  ratio  be- 
tween iron  and  copper  losses  is  somewhat  higher  in  turbine 
generators  than  in  ordinary  designs.  This  means,  in 
effect,  that  a  turbine  generator  is  one  having  relatively 
small  copper  losses,  and,  consequently,  one  in  which  the 


GENERATORS. 


77 


overload  capacity  and  overload  efficiency  are  high,  which 
is  found  to  be  the  case. 


Fig.  51. 

Considerations  of  strength  and  of  perfection  of  balance 
in  the  rotating  parts  are  of  prime  importance.  These  are 
obtained  by  the  choice  of  the  strongest  materials,  used  in 


78      POLYPHASE  APPARATUS  AND  SYSTEMS. 


GENERATORS.  79 

ample  sections,  and  by  the  exercise  of  the  greatest  care  in 
machining  the  parts.  Fig.  50  shows  one  form  of  construc- 
tion of  the  revolving  part  (in  this  case  the  field)  of  the 
turbine  alternators  manufactured  by  the  General  Electric 
Company.  In  this  type  the  revolving  part  is  built  up  of 
sheet  steel  laminae,  about  J  inch  thick,  symmetrically 
assembled  around  a  central  spider.  The  field  coils  em- 
body the  familiar  edgewise  winding  and  are  held  in  place 
by  the  projecting  pole  tips.  Wedge-shaped  retainers  are 
also  provided  between  the  sides  of  adjacent  spools,  these 
retainers  being  securely  keyed  in  place. 

In  the  stationary  element  the  armature  coils  are  placed, 
in  the  customary  way,  in  the  slots  with  which  the  core  is 
provided,  and  supported  by  an  arch-like  binding  band 
structure  at  the  ends.  This  alternator,  conforming  to  the 
turbine  standards  of  the  General  Electric  Company  for  all 
except  the  smaller  sizes,  is  of  the  vertical  shaft  type,  the 
shaft  being  supported  by  a  step  bearing  in  the  base  of  the 
turbine,  which  is  located  beneath  the  generator.  Fig.  51 
is  representative  of  the  type  of  Curtis  turbine  set  manu- 
factured by  this  company. 

The  turbine  alternators  manufactured  by  Parsons,  West- 
inghouse,  Brown- Boveri  and  most  other  manufacturers, 
employ  the  horizontal-shaft  design.  In  the  machines 
manufactured  by  these  companies  the  rotor  is  usually 
machined  from  a  steel  casting,  the  poles  being  solid  and 
the  armatures  being  of  the  closed  slot  type  with  the  wind- 
ings threaded  in  by  hand.  Fig.  52  shows  a  typical  turbo- 
alternator  of  the  Parsons  type  as  constructed  by  the  West- 
inghouse  Electric  and  Manufacturing  Company.  Figs.  53 
and  54  illustrate  respectively  the  stationary  armature  and 
the  revolving  field  of  a  similar  alternator. 


8o 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Conditions  Affecting  Cost.  —  From  what  has  preceded  it 
will  be  easily  understood  that  the  first  factor  in  determin- 
ing the  cost  of  a  polyphase  generator  of  given  capacity 
and  conditions  of  operation  is  its  speed.  The  initial 


Fig.  53. 


voltage  is  another  factor,  and  likewise  the  efficiency,  fre- 
quency, the  regulation,  and  the  temperature  rise.  A  gen- 
erator wound  •  for  high  voltage  may,  under  certain  condi- 
tions of  proportion  and  design,  cost  more  to  construct 
than  a  low  voltage  machine  of  the  same  characteristics 


GENERATORS.  8 1 

together  with  a  complement  of  step- up  transformers.  A 
generator  of  high  efficiency  can  be  built  at  a  reasonable 
cost  but  at  some  sacrifice  in  regulation.  Conversely,  the 
same  generator  may  be  designed  for  better  regulation  at 
the  sacrifice  of  efficiency,  and  cost  no  more.  This  is  for 
the  reason  that  machines  having  close  regulation  must  be 
designed  with  small  armature  reaction,  which  means  the 
use  of  liberal  iron  sections  and  increased  cost,  or  on  the 
other  hand,  with  high  iron  densities,  which  means  larger 
core  losses  and  excitation  losses  and  consequently  a  poorer 


Fig,  54. 

efficiency.  To  obtain  both  these  constants  in  an  eminent 
degree  requires  a  liberal  use  of  both  copper  and  iron  and 
results  in  an  expensive  machine. 

The  frequency  for  which  a  generator  is  designed  also 
influences  the  cost.  For  a  given  output  and  speed  and  for 
the  same  constants  of  operation,  low  frequency  machines 
will  require  considerably  more  copper  in  the  armature  and 
somewhat  less  copper  in  the  field,  the  total  copper  required 
in  the  machine  being  practically  the  same  in  either  case. 
The  labor  item  is  usually  lower  in  a  low  frequency  ma- 
chine because  on  the  average  it  costs  less  to  wind,  say,  ten 


82 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


large  coils  than  twenty  small  ones,  even  though  the  weight 
is  the  same  in  either  case.  On  the  other  hand,  the  amount 
of  armature  iron  required  in  low  frequency  generators  is 
larger,  because  with  the  diminution  in  the  number  of 


4UU 


800 


200 


100 


10  20  30  40 

Per  Cent  Reduction  in.  Cost 
Fig.  55. 


50 


60 


poles  the  amount  of  ftux  to  be  carried  by  the  armature  is 
split  up  into  fewer  paths  and  the  cross  section  must  conse- 
quently be  proportionately  larger.  This  condition  reaches 
its  extreme  in  the  case  of  bi-polar  alternators,  such  as 
those  of  medium  capacity  for  direct  connection  to  steam 


GENERATORS,  83 

turbines,  where  the  section  of  armature  iron  must  be  suffi- 
cient to  carry  one-half  the  entire  flux  generated  by  the 
field  winding.  Iron,  however,  is  cheap  as  compared  with 
ihe  price  of  labor,  so  that  on  the  whole  a  machine  of  low 
frequency  will  cost  somewhat  less,  other  things  being 
equal,  than  one  of  high  frequency.  The  proportionate 
saving  incident  to  reduction  in  frequency  is  more  notice- 
able  in  the  lower  speeds,  such  as  a^re  used  with  direct- 
connected  engine-driven  units.  The  differences  are  not 
great  at  the  medium  speeds,  or  in  belt-driven  generators, 
or  in  generators  that  are  provided  with  parts  that  remain 
the  same  irrespective  of  the  frequency.  In  the  highest 
speeds  the  low  frequency  machines  will  be  heavier  and 
more  costly,  largely  by  reason  of  the  extraordinary  amount 
of  armature  iron  which  has  to  be  provided  where  the 
number  of  poles  is  very  low. 

Fig.  55  shows  in  an  approximate  degree  the  relative 
reduction  in  cost  with  increasing  speed.  In  using  this 
curve  for  comparison  of  costs,  it  must  be  kept  in  mind  that 
it  is  only  approximately  correct  and  applies  to  generators 
of  the  same  type,  frequency,  general  constants,  and  condi- 
tions of  operation,  and  that  it  does  not  extend  far  enough  to 
show  the  point  at  which,  as,  for  example,  with  high  speed 
turbo-alternators,  an  increase  rather  than  a  decrease  in 
cost  is  involved  by  further  increase  of  speed. 


Sd      POLYPHASE  APPARATUS  AND  SYSTEMS. 


CHAPTER    IV. 
INDUCTION    MOTORS. 

Principles  of  Operation The  induction  motor  can  be 

compared  to  a  direct-current  shunt  motor,  the  essential 
difference  being  that  the  armature  or  working  current  of 
the  shunt  motor  is  led  into  it  by  brushes,  while  the  work- 
ing current  of  the  induction  motor  is  an  induced  or  trans- 
former current.  The  windings  of  the  induction  motor, 
connected  to  the  supplying  circuit,  besides  carrying  the 
exciting  current,  have  the  additional  function  of  supply- 
ing the  transformer  current.  The  induction  motor  is  thus 
seen  to  combine  the  principles  of  operation  of  both  a 
motor  and  a  transformer.  Rotation  may  be  considered  as 
being  produced  by  the  revolving  member  following  a  shift- 
ing magnetic  field  which  is  the  resultant  of  two  or  more 
alternating  fields  differing  in  phase.  The  explanation  of 
the  working  of  the  induction  motor  by  reference  to  the 
rotating  magnetic  field  alone,  however,  is  apt  to  mislead 
and  to  hide  its  true  functions. 

The  two  elements  of  an  induction  motor  are  preferably 
designated  as  primary  and  secondary,  and  sometimes  as 
field  and  armature.  Either  may  be  indifferently  the  rotor 
or  stator.  ^  » 

When  running  without  load,  the  rotor  speed  is  very 
closely  that  of  the  rotating  field,  and  there  is  a  very 
small  current  induced  in  the  secondary.  The  magnetic 


INDUCTION    MOTORS.  85 

pull  of  this  current  on  the  field  produces  a  feeble  torque. 
The  current  taken'  by  the  primary  member,  or  field,  is 
then  composed  of  the  magnetizing  current  and  that  re- 
quired for  overcoming  magnetic  and  mechanical  friction. 
As  the  power  factor  is  low  at  light  loads,  being  not  more 
than  15  or  20  per  cent  in  most  commercial  motors,  the 
energy  supplied  is  not  much  greater  than  that  consumed 
by  a  shunt  motor  of  the  same  capacity. 

When  running  under  load,  the  speed  of  the  revolving 
element  falls  away  from  that  of  synchronism,  and  the 
E.M.F.  and  working  current  induced  by  the  relative 
cutting  of  the  lines  of  force,  increase  with  the  difference 
in  speeds.  The  pull  of  this  increased  current  on  the  field 
produces  a  powerful  torque.  The  departure  from  the 
speed  of  synchronism  is  called  the  "slip,"  and,  within  cer- 
tain limits,  is  proportional  to  the  total  secondary  resistance. 

To  insure  high  efficiency  and  good  regulation,  the  resist- 
ance of  the  shunt  motor  armature  must  be  kept  as  low  as 
practicable.  For  the  same  reason,  the  windings  of  the 
secondary  of  the  induction  motor  should  have  a  low  re- 
sistance. 

Methods  of  Starting  Motors On  connecting  an  induc- 
tion motor  to  its  supplying  circuit,  there  is  an  excessive 
rush  of  current,  which  can  be  prevented  only  by  the  use  of 
some  device  external  to  the  motor  windings  proper.  There 
are  a  number  of  such  arrangements  for  reducing  the  start- 
ing current  of  motors. 

One,  and  probably  the  most  common  device,  consists 
essentially  of  a  variable  resistance,  which  can  be  cut  in 
or  out  of  circuit  with  the  secondary  winding.  When  the 
secondary  element  is  the  rotor,  this  resistance  often  occu- 
pies a  space  within  the  armature  spider.  When  so  located 


86      POLYPHASE  APPARATUS  AND  SYSTEMS. 

it  may  consist  of  copper  strips,  or  —  as  is  usually  the 
case  —  of  iron  cast  into  a  compact  grid  form,  having  a 
number  of  contact  points.  The  whole  of  this  resistance  is 
in  series  with  the  secondary  winding  at  starting.  As  the 
motor  attains  speed,  a  circular  short-circuiting  switch, 
mounted  in  a  ring  encircling  the  shaft,  is  pushed  centrally 
by  a  lever,  thus  cutting  out  the  resistance  in  as  many 
successive  steps  as  there  are  contact  points.  Motors  pro- 
vided with  this  starting  device  are  usually  designed  to 
start  with  a  torque  ranging  from  75  to  150  per  cent  of 
full-load  torque.  This  motor  has  the  desirable  charac- 
teristic that  the  current  is  very  nearly  proportional  to  the 
torque  from  starting  to  full-load  speed. 

The  rheostat  is  frequently  external  to  the  motor.  With 
this  arrangement,  when  the  secondary  revolves,  collector 
rings  are  required  to  convey  the  induced  current  to  the 
rheostat.  When  the  primary  is  the  revolving  element, 
collector  rings  are  also  needed  to  supply  the  main  current 
to  the  motor. 

A  water  rheostat  is  sometimes  employed,  by  means  of 
which  the  induced  current  is  varied,  its  strength  varying 
with  the  depth  to  which  the  plates  are  immersed.  With 
this  device,  the  current  taken  by  the  motor  is  closely 
proportional  to  the  torque,  from  starting  to  full-load 
speed. 

Another  method  of  starting  induction  motors  consists 
in  reducing  the  impressed  volts  by  the  use  of  some  form  of 
reactance  or  of  compensator  coils,  or  of  resistance  in  the 
main  circuit.  A  compensator  is  the  most  efficient  means 
of  cutting  down  the  voltage,  and  the  most  generally  em- 
ployed, one  coil  being  required  for  each  phase. 

The  connections  of  a  popular  starting  device  for  two- 


INDUCTION    MOTORS.  87 

phase  motors  are  shown  in  Fig.  56.  Two  compensator 
coils,  or  auto-transformers,  are  provided,  one  for  each 
phase.  A  cylindrical  switch,  like  that  in  the  familiar 
street  car  controller,  having  segments  of  proper  width 
and  spacing,  acting  in  conjunction  with  a  set  of  stationary 
contact  fingers  (M,  Lv  Av  A2,  etc.),  effects  the  proper 


TO  2   PHASE  MOTOF 


Fig.  56. 


circuit  combinations  during  starting.  The  salient  feature 
of  this  type  of  starting  device  is  that  the  voltage  is  applied 
to  the  motor  in  three  steps,  first  a  low  potential,  then  a 
medium  potential,  and  finally  full  potential.  Additional 
taps  are  brought  out  from  the  compensator  coils  for  the 
purpose  of  varying  the  values  of  the  two  starting  voltages. 

JThe  connections  of  a  starting  compensator  for  a  three- 
phase  motor,  as  made  by  another  American  company,  are 


88 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


shown  in  Fig.  57.  In  this  design  there  is  but  one  step 
between  the  "off"  and  the  "running"  position.  The 
necessary  circuit  combinations  are  effected  by  cylindrical 
switch  segments  and  stationary  contacts  of  usual  form, 
and  extra  taps  are  provided  in  the  compensator  coils  for 
varying  the  starting  voltage.  In  this  make  of  compen- 
sator, when  used  with  motors  of  15  H.P.  and  under,  there 
are  three  taps  with  voltages  40  per  cent,  60  per  cent,  and 
80  per  cent  of  full  voltage.  For  motors  above  15  H.P. 
four  taps  are  provided,  at  40  per  cent,  58  per  cent,  70  per 


Fig.  57. 

cent,  and  85  per  cent  of  full  voltage.  The  selection  of 
the  proper  tap  voltages  is  determined  by  trial,  according 
to  the  amount  of  starting  torque  necessary  for  the  motor 
to  have  under  operating  conditions,  and  when  once 
adjusted  no  change  is  required.  In  both  types  the 
compensator  switch  serves  also  as  main  line  switch,  the 
motor  terminals  being  "dead"  when  the  compensator 
is  in  the  "off"  position.  Separate  line  switches  are 
therefore  unnecessary. 

In  both  the  diagrams  it  will  be  noticed  that  two  leads 
are  provided  in  each  phase  on  the  compensator  connection 


INDUCTION   MOTORS.  89 

board.  This  is  for  the  purpose  of  by-passing  the  fuses  so 
that  the  possible  large  currents  taken  at  starting  will  not 
cause  the  automatic  cut-outs  to  act,  the  circuit  being  so 
arranged  that  when  the  compensator  is  in  the  running 
position  the  fuse  is  cut  into  circuit.  The  switch  connec- 
tions are  such  that  the  compensator  coils  are  disconnected 
as  so  m  as  the  running  position  is  reached.  This  is  done 
not  only  to  prevent  needless  waste  of  energy  in  the  form 
of  iron  and  copper  losses  in  the  cores  and  windings  of  the 
compensator,  but  also  to  prevent  overheating  of  the  device. 
Since  the  coils  are  intended  to  be  in  circuit  only  during 
the  short  periods  corresponding  to  the  time  required  for 
starting,  they  do  not  need  to  be  designed  to  radiate  con- 
tinuously the  amount  of  heat  represented  by  the  com- 
bined core  losses  and  PR  losses.  With  this  in  view  it 
is  possible  during  the  starting  periods  to  work  both  the 
iron  and  copper  at  very  high  densities  such  as  would 
result  in  a  burnout  were  the  coils  left  continuously  in 
circuit.  It  is  accordingly  necessary  to  disconnect  them 
from  the  circuit  as  soon  as  the  motor  has  reached  full 
speed,  and  the  connections  of  the  starting  switch  are 
devised  accordingly. 

Induction  motors  which  are  put  in  operation  by  the 
first  method,  may  be  designated  as  the  variable  resistance- 
in-armature  type.  They  frequently  have  a  higher  self- 
induction,  and  in  the  rotor  require  more  copper  and  more 
iron.  The  secondary  winding  is  definite  and  polar,  and 
the  additional  iron  in  the  rotor  is  necessitated  by  the  fact 
that,  owing  to  the  space  taken  up  by  the  insulation  of  the 
rotor  bars,  the  slots  are  wider  and  deeper,  which  results  in 
a  greater  total  depth  of  iron.  The  polar  winding  indi- 
cates also  the  reason  for  the  higher  self-induction  of  this 


90      POLYPHASE  APPARATUS  AND  SYSTEMS. 

type,  which  is  remediable  by  increasing  the  number  of 
slots  —  in  other  words,  by  making  the  rotor  winding  as 
distributed  as  possible.  This  feature,  in  turn,  narrows 
the  teeth  slightly,  and  by  thus  increasing  the  density, 
tends  to  enhance  the  core  loss  and  to  diminish  the  power 
factor.  A  low  total  resistance  being  necessary  in  order  to 
keep  down  the  slip,  it  follows  that  a  liberal  cross  section 
must  be  chosen  for  the  rotor  windings  of  this  type  by 
reason  of  the  increased  length  of  turn  which  the  longer 
end  connections  impose.  The  total  rotor  copper  is  there- 
fore large,  both  by  reason  of  the  total  length  of  copper  em- 
ployed and  by  reason  of  its  large  cross  section. 

Motors  which  are  used  with  the  compensator  starter 
may  be  designated  as  the  compensator,  or  short-circuited 
armature  type.  Their  distinctive  feature  is  the  short- 
circuited  armature,  which  is  usually  of  the  squirrel-cage 
construction.  This  construction  gives  a  low  self-induction 
to  the  rotor  and  thus  to  the  whole  motor,  thereby  insuring 
large  maximum  output.  In  this  type  the  amount  of  rotor 
iron  is  a  minimum,  and  the  core  losses  small  by  reason  of 
the  narrower  slots  that  can  be  used.  The  end  connections 
are  extremely  short,  and  the  cross  section  of  rotor  bar 
small,  so  as  to  provide  sufficient  resistance  to  insure  fair 
starting  torque,  so  that  the  rotor  copper  as  a  whole  is  of 
small  amount. 

In  either  type  the  construction  of  the  stator  is  prac- 
tically the  same,  both  as  to  amount  of  material  and  to 
design. 

In  starting  an  induction  motor  with  variable  secondary 
resistance,  precaution  must  be  taken  that  the  resistance 
is  all  in,  otherwise  the  flow  of  current  may  overheat  the 
motor  or  overload  the  lines.  The  armature  lever  should 


INDUCTION    MOTORS.  £1 

be  pulled  out  as  far  as  it  will  go;  then  the  line  switch  may 
be  closed,  and,  finally,  the  short-circuiting  switch  may  be 
slowly  closed.  The  motor  should  be  handled  at  starting 
to  reach  full  speed  in  about  fifteen  seconds.  As  the 
secondary  resistance  is  of  a  capacity  only  to  start  the 
motor,  it  never  should  be  left  in  circuit  or  used  to  regulate 
the  speed  of  the  motor.  The  motor  is  shut  down  by  first 
opening  the  line  switch  and  then  open-circuiting  the  arma- 
ture resistance. 

As  the  drop  in  a  good  transformer  on  a  lighting  load 
is  within  3  per  cent,  and  on  an  inductive  load,  as  motors, 
seldom  less  than  5  per  cent,  it  is  advisable  to  always  use 
separate  transformers  for  lights  and  for  motors.  The 
exception  to  this  rule  is  in  a  secondary  system  of  distribu- 
tion, where  the  motor  load  is  a  proportionately  small  part 
of  the  entire  load. 

Induction  motors  are  sometimes  started  by  being  con- 
nected directly  to  the  supplying  circuit  without  the  use  of 
any  form  of  starting  device.  Such  a  motor  will,  of  course, 
take  a  large  starting  current.  This  can  be  kept  down  by 
making  the  resistance  of  the  armature  conductors  rather 
high,  and  by  confining  the  motor  to  work  requiring  a  small 
starting  torque.  A  motor  started  in  this  way  should  not  be 
used  on  circuits  where  the  effect  of  a  large  starting  current 
on  the  potential  regulation  of  the  system  is  of  importance. 

The  direction  of  rotation  of  a  three-phase  induction 
motor  is  reversed  by  reversing  any  two  of  the  leads,  and 
of  a  two-phase  motor  by  changing  the  two  leads  of  either 
phase. 

Construction  of  Primary  and  Secondary.  —  The  simple 
and  substantial  construction  of  the  induction  motor  is  one 
of  its  chief  advantages,  resulting  in  a  minimum  cost  of 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Fig.  58. 


maintenance  and  attendance.     While  either  element  may 
be  the  rotor,    by   far   the  larger   number   of  commercial 


INDUCTION    MOTORS. 


93 


motors   are   now   constructed   with   a   fixed   primary   and 
with  a  rotor  secondary. 

The  fixed  primary  is  similar  in  construction  to  the  sta- 
tionary armature  of  a  revolving  field  generator  with  multi- 
tooth  winding. 


Fig.  59. 

It  is  built  up  of  slotted  laminations  mounted  on  a  cast- 
iron  spider.  The  coils  are  imbedded  in  the  slots.  Fig.  58 
illustrates  the  primary  or  field  ready  to  receive  its  con- 


94      POLYPHASE  APPARATUS  AND  SYSTEMS. 

ductors.  These  stationary  windings  are  usually  protected 
from  mechanical  injury  by  end  shields,  which  frequently 
support  the  bearings.  The  Westinghouse  Company  em- 
ploy this  form  of  construction  in  even  the  largest  sizes,  as 
illustrated  in  Fig.  59,  which  represents  an  800  H.P.  motor 
of  the  short-circuited  armature  type. 

This  motor  is  wound  for  60  cycles  and  2000  volts,  it 
has  36  poles,  hence  a  synchronous  speed  of  200  revolu- 
tions per  minute. 

In  most  motors  made  in  this  country  the  stator  punch- 
ings  are  provided  with  open  slots  (as  shown  in  Fig.  58) 
and  use  form  wound  coils,  a  construction  which  reduces 
the  labor  of  winding  and  which  enhances  greatly  the  con- 
venience of  repair.  The  closed  slot  construction  however 
decreases  the  reluctance  of  the  magnetic  circuit  and  thus, 
by  reducing  the  magnetizing  current,  betters  the  power 
factor.  This  construction  is  widely  used  abroad  where 
lower  labor  costs  offset  the  extra  expense  of  hand  winding. 
It  is  also  employed  in  special  instances  in  this  country 
where,  for  example,  in  high  voltage  motors  the  width  of 
slot  demanded  by  the  insulation  would  in  the  open-slot 
construction  prevent  the  attainment  of  satisfactory  power 
factors  except  at  prohibitive  expense  in  other  directions. 

The  rotor  armature  of  the  standard  form  of  motor  has 
a  laminated  slotted  structure  similar  to  the  primary.  In 
motors  of  the  variable  resistance  type,  the  secondary  has 
a  definite  series  of  coil  windings,  corresponding  to  the  polar 
windings  of  the  primary.  Since  motors  of  the  variable 
resistance  type  may  have  the  resistance  internal  to  the 
rotor  and  revolving  with  it,  or  external  to  the  rotor,  the 
rotor  windings  in  the  latter  case  are  brought  out  to  slip 
rings  from  which  are  led  by  brushes  the  currents  passing 


INDUCTION   MOTORS. 


95 


to  the  stationary  rheostat.  Motors  of  the  short-circuited 
type  are  generally  wound  with  copper  bars  laid  in  the 
slots  and  connected  at  both  ends  by  short-circuiting  metal 
rings.  Secondaries  of  this  construction  are  termed  squirrel- 
cage  armatures. 

Fig.  60  shows  the  elements  of  a  50  H.P.  motor  of  a 
standard  make,  the  photograph  illustrating  the  stationary 
primary  or  stator  and  showing  the  variable  resistance 
type  armature  with  collector  rings  for  external  rheostat. 


Fig.  60. 

In  the  motor  at  one  time  manufactured  by  the  Stanley 
Company  (Fig.  61)  the  field  is  stationary.  There  are,  in 
reality,  two  fields  and  two  armatures.  The  secondary 
windings  are  connected  so  that  the  wire  lying  under  the 
field  poles  on  one  armature  is  in  series  with  the  wire  lying 
between  the  poles  on  the  other.  The  field  coils  are  stag- 
gered, each  half  alternately  playing  the  part  of  a  motor 
and  transformer. 

Starting  Torque  and  Current.  —  At  normal  voltage  cer- 
tain types  of  motors  possessing  a  moderate  secondary 


OF    THE 
IIMIWCDQITY 


96 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


resistance  —  as,  for  instance,  a  motor  of  the  variable 
resistance  type,  with  the  resistance  cut  out  —  will  have  a 
small  starting  torque  due  to  the  reaction  on  the  primary 
of  the  excessive  induced  secondary  current.  The  starting 
current  consumed  by  the  motor  will  likewise  be  excessive. 


Fig.  61. 

At  nearly  synchronous  speed  such  a  motor  will  have  a 
powerful  torque.  By  increasing  the  secondary  resistance, 
the  starting  torque  is  raised  until  a  critical  resistance  is 
reached,  beyond  which  point  the  starting  torque  decreases. 
The  starting  torque  of  an  induction  motor  is  also  de- 
pendent upon  the  potential  applied  at  its  terminals.  The 


INDUCTION    MOTORS. 


97 


starting  current  is  reduced  by  lowering  the  voltage,  but  at 
the  sacrifice  of  the  torque  at  starting,  which  varies  as  the 
square  of  the  impressed  voltage. 

An  inspection  of  the  curves  in  Fig.  62  will  show  how 
the  starting  torque  is  influenced  by  varying  the  secondary 
resistance.  The  secondary  winding  of  the  motor  is 


Standstill 


Armature  Slip 
Fig.  62. 


Synchronism 


assumed  to  have  a  fixed  resistance  of  0.02  ohm.  At  start- 
ing, a  variable  resistance  is  connected  in  series,  making  a 
total  of  o.i  8  ohm.  The  corresponding  torque  is  about  25 
pounds,  or  150  per  cent  of  full-load  torque.  When  the 
motor  reaches  about  50  per  cent  of  synchronism,  part  of 
the  resistance  is  cut  out,  making  the  total  0.045  ohm.  The 
torque  now  increases  until  about  85  per  cent  of  synchro- 


98 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


nous  speed  is  reached,  when  it  begins  to  drop.  At  this 
point  the  remaining  resistance  is  short-circuited,  leaving 
only  the  resistance  of  the  secondary.  The  torque,  due  to 
this  resistance,  0.02  ohm,  reaches  its  maximum  at  about 
90  per  cent  of  synchronism.  The  starting  torque,  with  a 
secondary  resistance  of  0.02  ohm,  is  about  7  pounds. 
The  starting  torque,  due  to  a  resistance  of  0.75  ohm,  is  less 
than  when  the  total  secondary  resistance  is  0.18  ohm,  be- 
ing only  1 6  pounds.  The  current  in  the  primary  of  such 


I. 


-500- 


100-  -    10 


20      40 


60      80     100    120    HO    100     ISO    200     220    240     2(if>     ISO    300    320    340    360 
Horse  Power  O.utput 

Fig.  63. 

a  motor  at  all  speeds  will  be  nearly  proportional  to  the 
torque  developed.  At  the  moment  of  cutting  out  the  suc- 
cessive resistances,  the  current  will  momentarily  increase  in 
strength.  It  can  be  readily  seen  that,  by  using  a  sufficient 
number  of  resistance  steps,  the  motor  could  be  brought  up 
to  speed  with  uniform  torque  and  current.  When  the 
motor  is  taxed  beyond  its  capacity,  its  torque  and  speed 
rapidly  diminish  and  a  large  current  will  flow.  This  break- 
down point  is  determined  by  the  design  of  the  motor,  and 


INDUCTION   MOTORS.  99 

is  usually  chosen  at  from  50  to  100  per  cent  greater  than  the 
rated  load.  The  working  point  of  such  a  motor  is  on  the 
descending  portion  of  the  power  curve,  at  about  two-thirds 
of  the  maximum  output.  Curves  of  torque  and  amperes 
input  at  all  loads,  of  a  175  H.P.  motor,  are  given  in  Fig.  63. 
The  maximum  torque  which  a  given  motor  can  deliver 
is  a  constant.  The  speed  at  which  this  maximum  torque 
occurs,  however,  depends  upon  the  value  of  the  rotor 
circuit  resistance,  as  shown  by  the  curves  in  Fig.  62. 

The  magnetizing  current  which  is  characteristic  of  most 
alternating-current  apparatus,  such  as  transformers,  induc- 
tion motors,  etc.,  has  the  effect  of  increasing  the  full-load 
current  and  putting  a  greater  demand  on  transformers, 
line,  and  generators.  The  total  current  is  greater  than 
that  actually  required  in  supplying  the  losses  and  doing  the 
work  of  the  motor.  The  ratio  of  this  working  or  energy 
current  to  the  total  current  gives  the  power  factor. 

As  the  starting  current  of  the  motor,  with  short-circuited 
armature,  is  reduced  by  lowering  the  voltage,  it  follows 
that,  for  the  same  starting  torque  as  that  developed  by  the 
variable  resistance  type,  the  current  will  be  considerably 
greater. 

The  line  starting  current  and  the  torque  of  some  makes 
of  motors  with  short-circuited  armatures,  expressed  in  per- 
centages of  full  load,  are  about  as  follows: 

STARTING  CURRENT       STARTING 
JJ"M-*«          FROM  LINE.  TORQUE. 

40%  112%  32% 

60%  250%  72% 

80%  45°%  I28% 

100%  700%  200% 

The  local  current  between  the  compensator  and  motor 
will  be  greater  than  the  line  starting  current,  as  its  poten- 


IOO 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


tial  is  lower.     The  action  of  the  compensator  is  similar  to 
that  of  a  transformer. 

By  increasing  the  resistance  of  the  armature  of  these 
motors,  the  starting  current  for  the  same  torque  is  de- 
creased; but  the  result  is  an  increase  in  the  slip,  and  a  loss 
of  efficiency  which  may  be  as  great  as  2  or  3  per  cent. 


juble  Throw-Switch 

1-0  r-0 


Motor 


Generate 


Doub 

Running  Side 

—i       Starting  Side 

le  Throw-Switch 

IP 

j 

Motor 

-50 

t 

KA/wj 

L 

T 

,i 

Two 
Transformers 

UJ 

wvwv 

J 

Fig.  64  and  Fig.  65. 

Where  transformers  arc  used  for  individual  motors,  or 
where  several  motors  are  located  close  to,  and  operated 
from,  a  bank  of  transformers,  it  is  sometimes  practical  to 
bring  out  taps  from  the  secondary  winding,  and  use  a 
double  throw  motor  switch,  thereby  making  provision  for 
starting  the  motor  at  low  voltage,  and  saving  the  cost  of 
a  compensator.  The  connections  of  such  an  arrangement 
are  shown  in  Fig.  64  and  Fig.  65. 


INDUCTION    MOTORS. 


101 


Motors  of  the  variable-resistance  type  have  a  special 
range  of  usefulness  when  operated  from  circuits  requiring 
good  regulation  such  as  is  demanded  in  central  station 
work.  They  are  desirable  for  service  where  high  starting 
torque  at  moderate  current  input  is  required. 


Fig.  66. 

Fig.  66  represents  a  50  H.P.  three-phase  motor  of  this 
type,  and  shows  at  the  left  the  lever  which  actuates  the 
short-circuiting  switch  of  the  internal  resistance. 

The  short-circuited  type  is  to  be  recommended  for 
power  circuits,  and  when  the  motors  must  be  started  from 


102    POLYPHASE  APPARATUS  AND  SYSTEMS. 

a  distance  and  simplicity  of  operation  is  of  moment.  It  is 
adapted  for  service  calling  for  low  starting  efforts  and  con- 
tinuous running,  and  is  especially  advantageous  when  the 
motors  are  apt  to  run  overloaded,  or  on  circuits  of  unsteady 
voltage.  It  is  not  adapted  for  lighting  circuits,  where  good 
regulation  is  important,  unless  the  current  at  starting 
is  small  when  compared  with  the  capacity  of  feeders  and 
generators. 

Speed  Regulation.  —  Absolutely  synchronous  speed  is 
never  attained  in  an  induction  motor,  as  some  slip  is 
required  to  furnish  the  current  consumed  by  the  light- 
load  losses.  Under  increasing  load  the  speed  will  fall 
away  from  synchronism  until  the  break-down  point  is 
reached, "and  if  the  motor  is  not  relieved  of  its  load  it  will 
come  to  a  standstill.  The  current  will  then  be  at  its 
maximum.  The  fall  in  speed  from  that  at  light  load  to 
that  at  normal  rated  load  will  vary  in  some  types  of  induc- 
tion motors  from  ij  per  cent,  as  in  motors  of  100  H.P.,  to 
3  per  cent,  as  in  smaller  motors.  Motors  constructed 
with  high  and  fixed  secondary  resistances  may  drop  in 
speed  as  much  as  9  per  cent. 

The  complaint  has  been  made  against  the  induction 
motor  that  it  is  an  inflexible  piece  of  apparatus  in  respect 
to  regulation  of  speed.  It  is  quite  true  that  wide  varia- 
tions of  speed  are  obtained  in  modern  motors  only  at  the 
expense  of  efficiency  and  increased  cost  of  construction. 

There  are  a  number  of  methods  of  obtaining  a  variation 
of  speed  in  an  induction  motor. 

The  method  now  most  employed  is  that  by  rheostatic 
control.  A  resistance  is  intercalated  in  the  secondary 
circuit,  which  can  be  varied  by  short  successive  steps. 
The  range  of  speed  usually  demanded  of  a  variable  speed 


INDUCTION   MOTORS. 


103 


induction  motor  does  not  permit  the  use  of  the  small 
resistance,  such  as  is  used  in  starting  in  some  designs  of 
motor,  and  which  is  located  within  the  rotor  armature. 
An  external  rheostat  is  required  of  sufficient  size  to  dissi- 
pate a  considerable  amount  of  energy.  Fig.  67  shows  the 
connections  of  a  three-phase  motor  and  of  a  rheostatic  con- 
troller for  variable  speeds.  Collector  rings,  as  shown,  must 


Rheostat 


Forward 


Fig.  67. 

be  added  to  motors  having  revolving  secondaries  for  elec- 
trically connecting  the  windings  and  external  resistance. 

The  main  line  is  shown  as  passing  through  the  control- 
ler. By  this  arrangement  the  circuit  is  closed  simulta- 
neously with  the  commencement  of  the  operation  of  cutting, 
out  the  resistance.  It  is  in  appearance  similar  to  the 
well-known  street  car  controller,  and,  like  it,  is  reversible. 
Even  when  all  the  resistance  is  cut  out  it  will  be  seen 
that  there  is  still  some  loss  of  energy  in  the  cables  leading 


IO4     POLYPHASE  APPARATUS  AND  SYSTEMS. 

from  the  collector  rings  to  the  controller.  Where  the 
motor  is  to  run  for  considerable  periods  at  full  speed  and 
it  is  desired  to  eliminate  these  external  PR  losses,  a  sep- 
arate device  may  be  provided  to  short-circuit  the  collector 
rings  at  the  motor  after  the  external  rheostatic  resistance 
has  been  short-circuited.  Such  a  device  is  also  sometimes 
equipped  with  an  arrangement  which  at  the  same  time 
lifts  the  brushes  off  the  collector  rings,  thereby  eliminating 
also  the  small  but  constant  losses  which  would  other- 
wise exist  by  reason  of  PR  and  friction  in  the  brush 
contacts. 

When  two  motors  are  employed  together  in  the  same 
class  of  service,  as  for  instance  in  polyphase  railway  work, 
the  speed  may  be  reduced  one-half  and  less  by  the  method 
of  concatenated  or  tandem  control.  This  method  consists 
in  feeding  the  secondary  of  one  motor  which  is  connected 
to  the  supply  circuit  direct  to  the  primary  of  the  second 
motor.  The  motors  are  not  necessarily  of  the  same  con- 
struction, but  must  be  provided  with  collector  rings  and 
brushes.  The  secondary  current  of  the  first  motor  fur- 
nishes power  to  the  second  motor  instead  of  being  dissi- 
pated in  a  rheostat,  thus  directly  increasing  the  efficiency 
of  the  first  motor.  The  secondary  current  of  the  second 
motor  is  regulated  by  a  resistance  in  circuit.  A  speed 
lower  than  half  is  obtained  by  increasing  this  resistance. 
A  higher  speed  is  obtained  by  connecting  the  motors  in 
parallel  on  the  supply  circuit  with  their  secondaries  feeding 
into  regulating  resistances.  The  tandem  method  of  con- 
trol lowers  the  power  factor  of  the  first  motor  and,  thereby, 
its  torque.  The  speed-controlling  mechanism  is  somewhat 
complicated. 

A  water  rheostat  for  varying  the  secondary  resistance, 


INDUCTION    MOTORS. 


I05 


and,  correspondingly,  the  speed,  of  the  motor,  is  used  by 
Ganz  &  Go.  in  a  three-phase  railway  plant  in  Northern 
Italy.  The  general  form  of  this  rheostat  is  shown  in 
Fig.  68.  Compressed  air  is  admitted  below  the  bottom 
of  the  tank,  raising  it,  and  thus  increasing  the  depth  of 
immersion  of  the  metal  plates. 


Water  rheostat 


Fig.  68. 


A  more  elaborate  construction  of  the  water  rheostat  is 
employed  with  large  stationary  motors,  as  for  instance  in 
large  hoisting  work,  where  the  magnitude  of  the  energy  to 
be  dissipated  is  such  as  to  demand  special  arrangements 
for  cooling  the  rheostat.  In  this  form  the  tank  which 
contains  the  electrodes  and  the  solution  is  provided  with 
an  auxiliary  pump  which  continuously  circulates  the  elec- 


106     POLYPHASE  APPARATUS  AND  SYSTEMS. 

trolyte  through  coils  of  piping  cooled  by  water.  A  supply 
of  compressed  air  controls  the  height  of  the  liquid  and 
thus  the  depth  of  immersion  of  the  plates,  or  the  plates 
may  be  raised  and  lowered  manually.  A  liquid  rheostat 
of  this  type,  manufactured  by  the  Allgemeine  Elektricitats 
Gesellschaft,  is  used  with  a  large  induction  hoisting  motor, 
supplied  by  that  company,  the  connections  and  arrangement 
being  shown  diagrammatically  in  Fig.  69.  The  rheostat  is 
shown  at  D  in  the  lower  left  hand  of  the  diagram  together 
with  its  circulating  pump,  which  is  of  the  centrifugal  type. 
The  letters  on  the  diagram  refer  to  the  following  parts : 

A  Emergency  switch.  H  Depth  Indicator. 

B  High  tension  fuses.  J  Foot-controlled  emergency  brake. 

C  Reversing  switch.  K  Speed  varying  gear. 

D  Liquid  rheostat.  L  Lever  for  K. 

E  Main  controller  lever.  M  Electro-magnetic  brake. 

F  Controlling  lever  for  speed.        N  Transformer  for  supplying  current 

G  Brake  lever.  to  brake  and  to  pump  motor. 

Other  Methods  of  Speed  Variation.  —  The  speed  of  an 
induction  motor  can  also  be  controlled  by  changing  the 
impressed  voltage  at  the  motor.  This  method  requires  the 
use  of  an  external  reactance  or  a  compensator,  and  a 
motor  possessing  a  high  fixed  armature  resistance. 

The  controller  and  compensator  are  usually  separate. 
By  a  sufficient  number  of  taps  in  the  latter,  connected  by 
cables  to  the  controller,  a  graduated  variation  of  the  im- 
pressed voltage  is  obtained,  and  a  corresponding  variation  in 
speed.  This  method  finds  only  a  restricted  application 
and  is  even  then  usually  confined  to  motors  of  but  small 
capacity.  It  has  a  special  field  of  usefulness  where 
traveling  cranes  are  equipped  with  polyphase  motors, 
for  by  it  the  number  of  sliding  contacts  by  which  the 


INDUCTION    MOTORS. 


107 


current  is  led  to  the  hoisting  and   cross-travel   motors  is 
halved. 


Another  method  of  controlling  the  speed  is  by  chang- 
ing the  number  of  poles.  When  a  variety  of  speeds  is 
required,  this  method  is  complicated,  requiring,  in  addition 


loS 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


to  a  compensator,  an  elaborate  switching  device.  It  is 
objectionable  also,  as  the  motor  can  only  run  at  full,  one- 
half,  and  one-quarter  speed,  and  at  no  intermediate  speeds. 
This  method  has  been  successfully  employed  in  cases 
where  half  speed  and  full-load  torque  are  required,  and 
has  the  advantage,  not  possessed  by  other  methods,  that  it 
yields  a  fair  efficiency  at  half  speed,  there  being  no  waste- 
ful rheostat  losses. 

An  investigation  of  the  relative  efficiencies  and  power 
factors  of  induction  motors  of  10  H.P.  output,  equipped 
with  the  rheostatic  and  with  the  potential,  variable  speed- 
controlling  devices,  gives  the  approximate  results  shown 
in  the  following  table: 


SPEED. 

METHOD  OF 
CONTROL. 

EFFICIENCY. 

P.   F. 

AP.  EF. 

Full  

(   Rheostatic 

33 

86 

72 

I    Potential 

83 

86 

72 

Half 

\   Rheostatic 

41-5 

86 

36 

1   Potential 

36 

57 

20.5 

Quarter  . 

(    Rheostatic 
1    Potential 

21 

16 

86 
48 

18 
7-7 

The  torque  is  assumed  to  be  constant  at  all  speeds. 

In  practice  it  will  be  found  that,  in  order  to  give  the 
best  all-round  results,  the  motor  for  potential  control  will 
have  a  lower  efficiency  at  full  speed  than  the  motor  built 
for  rheostatic  speed  control. 

The  motor  with  rheostatic  control  shows  the  same  power 
factor  at  all  speeds. 

The  potential  control  gives  a  lower  power  factor  and 
efficiency  at  all  but  full  speed, 


INDUCTION    MOTORS, 


The  motor  controlled  by  change  of  poles  will  be  found 
to  be  the  most  efficient  for  half  and  quarter  speeds,  and 
has  the  highest  power  factor  except  at  quarter  speeds. 

Of  the  commercial  methods  of  obtaining  speed  variation, 
that  by  potential  control  is  inferior  to  the  rheostatic  con- 
trol in  point  of  efficiency.  A  drawback  to  the  rheostatic 
method  is  that  the  motor  requires  collector  rings. 

Ilgner  System.  —  A  special  application  of  the  speed  con- 
trol of  induction  motors  intended  to  run  with  a  large  but 
variable  slip  is  afforded  in  the  type  of  induction  motor  gen- 
erator set  used  in  the  Ilgner  system.  In  order  to  reduce  the 
fluctuations  of  load  incident  to  severe  duty  of  highly  inter- 
mittent character,  such  as  occurs  in  hoisting  work,  and  to 
secure  economical  results  in  current  consumption  at  frac- 
tional speed  running,  recourse  is  had  to  the  well-known  Ward 
Leonard  scheme  of  control.  By  this  scheme,  as  applied  to 
alternating-current  working,  the  hoist  motor  itself  is  of 
the  direct-current  type,  its  current  supply  being  derived 
from  a  motor  generate  set,  the  economy  of  the  system 
resulting  from  the  fact  that  a  variable  voltage  is  applied 
to  the  terminals  of  the  hoist  motor  by  means  of  variable 
excitation  on  the  direct-current  generator.  Full-load  cur- 
rent, and  thus  full-load  torque,  can  thus  be  imparted  to 
the  hoist-motor  at  any  desired  speed,  and  since  there  are 
no  rheostatic  losses  the  energy  taken  from  the  alternating- 
current  mains  is  proportional  only  to  the  useful  work  done. 
This  arrangement,  even  taking  into  account  the  fixed  losses 
in  the  motor  generator  set,  results  in  a  high  economy  of 
energy,  though  it  does  not  reduce  the  magnitude  of  the 
maximum  demand,  and  still  throws  back  on  the  feeders  or 
on  the  generating  station  -the  fluctuating  characteristics 
pertaining  to  hoisting  service. 


110     POLYPHASE  APPARATUS  AND  SYSTEMS. 

In  the  system  devised  by  Ilgner  the  motor  generator  set, 
the  direct-current  hoist  motor,  and  the  Ward  Leonard 
control  are  retained,  the  feature  of  novelty  being  the  in- 
troduction of  a  heavy  fly  wheel  as  part  of  the  motor  gen- 
erator; and  the  function  of  this  fly  wheel  is  to  absorb 
surplus  energy  at  times  of  minimum  demand  and  to  give 
it  out  at  times  of  maximum  demand,  in  this  way  equaliz- 
ing the  input  taken  by  the  motor  generator  set. 

Since  there  can  be  no  increment  nor  decrement  in  the 
energy  stored  in  a  rotating  mass  except  by  change  of 
speed,  and  since,  further,  the  speed  of  the  motor-generator 
set,  with  fly  wheel,  if  driven  by  a  motor  of  ordinary  design 
would  vary  only  within  narrow  limits  (as  fixed  by  the 
usual  magnitude  of  the  slip),  it  follows  that  the  slip  of  the 
motor  in  an  Ilgner  set  must  be  high  enough  to  insure  a 
speed  range  of  adequate  amount.  In  these  sets  the  maxi- 
mum slip  is  usually  fixed  at  about  16  per  cent,  and  as  the 
loss  of  energy  represented  by  this  degree  of  slip  is  consid- 
erable, an  external  rheostat  is  used,  commonly  of  the 
liquid  type  in  the  case  of  large  units,  the  rotor  of  the  induc- 
tion motor  being  polar  wound  and  equipped  with  collector 
rings  in  the  usual  manner. 

Fig.  70  illustrates  the  equalization  of  power  demand 
secured  through  the  application  of  this  system,  the  curves 
being  plotted  from  results  published  by  the  manufacturers. 
It  will  be  noted  that  although  the  output  demanded  from  the 
converter  ranged  between  30  H.P.  minimum  and  575  H.P. 
maximum,  the  variation  in  the  amount  of  power  absorbed 
by  the  induction  motor  was  only  between  187  H.P.  mini- 
mum and  310  H.P.  maximum.  In  the  apparatus  from 
which  these  curves  are  taken  the  external  resistance  in  the 
rotor  circuit  had  a  fixed  value.  A  more  perfect  equaliza- 


INDUCTION    MOTORS. 


Ill 


tion  of  the  input  curve  is  obtained  when  the  value  of  the 
external  resistance  is  automatically  varied.  A  modifica- 
tion of  the  water  rheostat  equipment  to  secure  this  employs 
a  special  controlling  motor,  the  current  input  to  which  is 
derived  from  series  transformers  intercalated  in  the  stator 
circuit  of  the  induction  motor  of  the  Ilgner  set.  The 
action  of  the  controlling  motor  is  so  arranged  that  when 


320fr 


Diagram  of  the  Winding  Motor  Oulpt 


Diagram  of  the  Ilgner  Converter  Output. 


-•  Output  at  Shaft  of  Converter.' 
.. .  *. .  —  Ontpnt  of  Three-Phase  Motor. 

_.,_..—  Power  taken  from  Three-Phase  Supply  Maint. 

Fig.  70. 


the  load  increases,  the  rheostat  plates  are  drawn  out  of  the 
liquid,  thus  increasing  the  rotor  resistance,  increasing  the 
slip,  throwing  more  load  on  the  fly  wheel,  and  thereby 
relieving  the  alternating-current  supply  system  of  the 
excess  power  demand. 

Since  the  equalization  of  power  consumption  achieved 
by  this  system  means  that  the  capacity  of  the  motor  ele- 
ment of  the  Ilgner  set  may  be,  and  is,  proportioned  to  the 
average  rather  than  to  the  maximum  demand,  the  motor 


112     POLYPHASE  APPARATUS  AND  SYSTEMS. 

element  will  always  be  smaller  than  the  generator  element. 
The  ratio  of  the  capacities  of  the  two  will  depend  princi- 
pally on  the  nature  of  the  load  cycle,  the  size  of  generator 
element  as  compared  with  that  of  the  motor  element  being 
greatest  in  a  service  calling  for  highly  intermittent  loads 
of  the  maximum  fluctuation.  This  is  strikingly  illus- 
trated in  the  case  of  the  installation  referred  to,  in  which 
the  motor  element  is  of  but  250  H.P.,  whereas  the  gener- 
ator element  is  proportioned  to  give  a  maximum  output  of 
650  kilowatts,  or  about  870  H.P.,  the  direct-current  hoist- 
ing motor  which  it  drives  having  a  normal  rating  of  350 
H.P.  In  other  words,  an  average  energy  supply  of  250 
H.P.  reinforced  by  the  stored  energy  of  the  fly  wheel 
suffices  for  a  duty  which  would  otherwise  impose  on  the 
supply  system  peak  loads  of  the  amount  represented  by 
the  maximum  output  of  the  generator  element.  In  order 
to  obtain  from  the  rotating  masses  the  amount  of  energy 
which  these  load  fluctuations  represent,  the  fly  wheel  must 
obviously  be  of  massive  construction  and  worked  at  high 
speed,  the  fly  wheel  of  the  converter  in  question  having, 
for  example,  a  weight  of  14,000  pounds  and  running  at  a 
peripheral  velocity  approaching  150  miles  per  hour. 


INDUCTION    MOTORS.  113 


CHAPTER  V. 
INDUCTION  MOTORS  (Concluded). 

Frequency.  —  Induction  motors  of  frequencies  of  from 
25  to  60  cycles,  as  constructed  at  the  present  time,  have 
somewhat  better  power  factors  and  efficiencies  than  higher 
frequency  motors.  Motors  of  a  frequency  of  125  cycles 
or  thereabouts  are  seldom  built  in  sizes  above  20  H.P. 
Motors  of  this  frequency  being  somewhat  difficult  of  con- 
struction on  account  of  the  small  air  gap  required  and  the 
greater  number  of  poles,  are  not  cheaper  than  25-cycle  or 
6o-cycle  motors  of  corresponding  sizes,  as  might  be  expected. 
The  reverse  holds  good  with  lower  frequencies,  6o-cycle 
motors  costing  less  to  build  than  motors  of  25  cycles. 

Twenty-five-cycle  motors  have  the  disadvantage  that  on 
account  of  difficulties  in  the  winding  construction,  the 
speeds  are  practically  limited  to  750,  500,  375,  and  300 
revolutions  per  minute.  The  bipolar  motor,  running  at 
1500  revolutions,  is  limited  to  the  smallest  sizes.  The 
slow  speeds  of  300  and  375  revolutions  make  the  motor, 
unless  it  be  one  of  great  capacity,  an  expensive  piece  of 
apparatus.  These  conditions  limit  the  average  practical 
speed  of  25-cycle  motors,  of  sizes  from  5  H.P.  to  75  H.P., 
to  750  revolutions. 

Frequencies  of  35  to  40  cycles  are  more  desirable  for 
the  average  conditions  of  motor  work,  as  they  permit  a 
much  greater  range  of  commercial  speeds. 


greater  than  the  full-load  output  will  give  only  ( -  -  j    X 


114     POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  frequency  of  60  cycles  likewise  permits  the  con- 
struction of  motors  with  a  wide  range  of  speed,  and  which 
are  comparatively  cheap  to  build  throughout  the  entire  list. 

Voltage.  —  Induction  motors  should  not  be  run  at  lower 
voltages  than  that  for  which  they  are  designed,  as  the  out- 
put varies  with  the  square  of  the  voltage.  For  instance, 
if  the  volts  at  the  motor  are  10  per  cent  lower  than  nor- 
mal, a  motor  which  has  a  maximum  output  of  30  per  cent 

\2 

Vioo 

130  =  105  per  cent  of  its  rated  output.  This  margin  is 
too  close  for  continuous  work,  as  it  will  not  take  care  of 
any  sudden  fluctuation  of  load  or  unusual  drop  in  the  line. 
The  output  of  the  motor,  on  a  higher  voltage  circuit  than 
that  for  which  it  is  designed,  will  be  increased,  and  the 
current  likewise,  especially  at  light  loads.  Within  ordi- 
nary variation  of  voltages,  the  power  factor  and  efficiency 
at  full  load  remain  practically  unchanged. 

In  laying  out  the  wiring  of  a  motor  which  takes  a  heavy 
starting  current,  allowance  should  be  made  for  this  mo- 
mentary current;  otherwise  the  impressed  volts  may  drop 
below  the  point  where  the  motor  will  start. 

Motors  with  stationary  fields  could  be  wound  for  fairly 
high  voltage,  but  for  the  distributed  form  of  winding  re- 
quired to  keep  down  self-induction,  the  space  necessary 
for  high  insulation  being  occupied  by  the  conductor. 
Standard  American  motors  below  50  H.P.  are  not  wound 
above  550  volts.  It  is  considered  practical  to  wind  larger 
motors  up  to  3000  volts  or  higher.  European  makers,  on 
the  other  hand,  build  motors  of  10  H.P.  to  30  H.P.  for 
pressures  of  500  to  2000  volts,  motors  of  50  H.P.  for  3000 
volts,  and  those  of  75  H.P.  and  larger  for  5000  volts, 


INDUCTION    MOTORS.  115 

Power  Factor  —  Efficiency.  —  It  has  been  seen  that  the 
ratio  of  the  energy  current  of  a  motor,  or  the  current 
required  in  supplying  its  losses  and  doing  the  work  to  the 
total  current  consumed,  gives  the  power  factor.  The  prod- 
uct of  the  power  factor  and  the  actual  efficiency  of  an  in- 
duction motor  gives  the  apparent  efficiency.  This  last 
quantity  determines  the  capacity  of  transformers  and  gen- 
erators required  for  supplying  current  to  the  motors.  As 
has  been  seen,  the  influence  of  the  power  factor  extends 
back  in  the  chain  of  transmission  with  greater  effect  on 
the  supplying  circuit,  necessitating,  in  the  case  of  a  poor 
power  factor,  on  account  of  its  inductive  effects,  an  addi- 
tional increase  in  the  capacity  of  the  transmission  lines. 
For  this  reason,  it  is  usually  of  importance  that  induction 
motors  be  designed  to  give  the  highest  possible  power  fac- 
tor. Where  the  generated  power  is  expensive,  it  is  some- 
times of  more  importance  to  use  motors  of  higher  efficiency 
than  those  of  high  power  factor.  Under  all  circumstances, 
however,  it  is  desirable  to  have  the  apparent  efficiency  of 
the  motors  as  high  as  possible. 

The  power  factors  of  standard  commercial  induction 
motors  of  American  manufacture  vary  at  full  load  from 
0.75  to  0.92,  depending  upon  the  size  and  frequency  of  the 
motor.  The  efficiencies  range  from  0.80  to  0.92.  The 
apparent  efficiencies  in  motors  above  5  H.P.  output  will 
be  found,  as  a  rule,  not  less  than  0.75.  This  means  that 
the  transformer,  supplying  current  to  induction  motors  of 
average  sizes,  must  have  a  capacity  of  i  kilowatt  for  every 
horse  power  output  of  the  motors. 

In  quarter-phase  motors  the  windings  are  in  90  degree 
relation,  and  as  a  rule  there  is  but  little  latitude  for  the 
designer  in  disposing  of  the  material  to  the  best  advan- 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


tage.  In  three-phase  motors  the  windings  are  in  60  de- 
gree relationship,  and  two  combinations  of  winding  are 
usually  available,  i.e.,  either  the  star  or  delta  connection 
of  coils.  This  circumstance  favors  the  three-phase  type 
somewhat,  and  with  equally  good  copper  arrangement  the 
three-phase  type  will  usually  show  a  slight  advantage  in 
power  factor,  ranging  from  i  per  cent,  where  the  power  fac- 
tor is  about  0.90,  to  about  3  per  cent  where  the  power  factor 
is  as  low  as  0.70,  these  values  being  taken  at  full  load. 

The  following  table  gives  approximate  capacities  of 
standard  transformers  that  should  be  used  with  two-phase 
and  three-phase  induction  motors: 


H.  P. 

CAPACITY  MOTOR. 

THREE-PHASE. 

TWO-PHASE. 
2  TRANSFORMERS. 

2  TRANSFORMERS. 

3  TRANSFORMERS. 

I 

,6  K.W. 

.5  K.W. 

.6  K.W. 

2 

i.       " 

.75    " 

i.       " 

3 

2.          " 

i         " 

1.5      « 

5 

3-          " 

2.           " 

3-       " 

1% 

4-       " 

2,5        « 

4.        " 

10 

c.        " 

*/ 

3-5      " 

5.       " 

'5 

7-5      " 

5-       " 

7-5  '  " 

20 

10.          " 

7-5      " 

10.          " 

30 

15.    « 

10. 

15.       " 

50 

25.    « 

15.        " 

25.       « 

75 

25.        " 

35-       " 

100 

30. 

45-       " 

The  efficiency  of  commercial  induction  motors  can  be 
somewhat  increased  by  not  sparing  iron  and  copper,  as 
the  losses  of  an  induction  motor  are  of  the  same  kind  as 
those  of  a  generator,  consisting  of  copper  loss,  hysteresis 
and  eddy  current  loss,  and  friction  loss. 

The  power  factor  can  be  bettered  by  reducing  the  air 


INDUCTION   MOTORS.  1 17 

gap  and  iron  density,  and  thereby  lowering  the  magnetiz- 
ing or  "wattless"  current.  To  do  this,  however,  and 
retain  high  efficiency,  increases  the  cost  of  the  motor,  and 
it  then  becomes  a  question  whether  the  increased  advan- 
tages are  worth  the  extra  expense.  Mechanical  considera- 
tions limit  the  clearance  between  field  and  armature.  Fig. 
63  shows  the  curves  of  efficiency,  power  factor,  and  appar- 
ent efficiency,  as  well  as  torque  and  ampere  input  of  a  175- 
H.P.  motor.  At  full  load  the  efficiency  is  91  per  cent,  the 
power  factor  .88,  and  the  apparent  efficiency  80  per  cent. 
The  efficiency  aj;  half  load  is  as  good  as  that  at  full  load; 
and  at  one-quarter  load  the  efficiency  is  still  well  up,  being 
85  per  cent.  The  break-down  point  is  at  about  twice  full 
load.  The  power  factor  is  highest  at  about  260  H.P., 
being  over  91  per  cent. 

In  many  cases  it  is  desirable  to  design  motors  so  that 
their  maximum  efficiency  occurs  at  about  three-quarters 
load.  This  is  especially  desirable  for  shop  work,  where 
the  driving  motors  are  called  upon  intermittently  to  give 
full  load,  the  average  demand  being  15  per  cent  to  30  per 
cent  less  than  the  load  for  which  they  are  rated. 

Condensers.  —  Condensers  are  used  to  improve  the 
power  factor  of  circuits  supplying  current  to  motors  by 
making  the  motors  take  current  in  proportion  to  the  loads. 
The  power  factor  of  the  motor  itself  is  not  affected,  but 
the  wattless  lagging  current  in  the  motor  is  offset  by  the 
leading  current  supplied  by  the  condensers,  and  its  per- 
nicious influence  confined  to  the  local  circuit  between  the 
condenser  and  the  motor.  Fig.  71  shows  the  apparent 
efficiency  of  a  Stanley  two-phase  motor  with  and  without  a 
condenser. 

The  condenser  consists  of  numerous  thin  sheet  conduc- 


Il8     POLYPHASE  APPARATUS  AND  SYSTEMS. 


tors,  separated  by  still  thinner  dielectrics,  the  whole  elec- 
trically connected  to  form  two  conductors.  As  the  size 
of  the  condenser  increases  rapidly  with  a  low  frequency 


10   2 

dns 


8       S        8        S5        §       8 
WMOfc/  tfJ/M0</  0W  AON3IOIJJ3 


and  voltage,  it  is  best  adapted  for  circuits  of  over  100 
cycles,  and  when  motors  are  used  for  not  less  than  500- 
volt  circuits. 


INDUCTION   MOTORS. 

Single-Phase  Motors.  —  Single- phase  induction  motors 
have  the  characteristic  form  of  polyphase  motors.  As  the 
flow  of  energy  in  the  single- phase  system  is  not  continuous, 
as  in  a  polyphase  system,  their  capacity  is  less  than  that  of 
polyphase  motors  of  the  same  dimensions.  In  respect  to 
torque,  power  factor  and  efficiency,  the  best  commercial 
motors  are  not  so  good  as  polyphase  motors.  An  external 
starting  arrangement,  sometimes  called  a  "  phase-splitter," 
is  sometimes  used  with  these  motors,  for  artificially  pro- 
ducing a  torque  sufficient  to  enable  them  to  start  from 
rest  under  a  partial  load. 

The  winding  of  a  two-pole,  single-phase  motor  is  shown 
in  Fig.  72.  It  has  a  two- phase,  interlinked  winding,  the 
common  terminals  being  at  III.  If  two  currents,  having  a 
difference  in  phase,  are  introduced,  the  dead  point  common 
to  all  single- phase  motors  will  be  overcome,  and  the  ar- 
mature will  revolve.  The  displaced  phase  is  produced  by 
a  combined  resistance  and  reactance  coil,  the  outline  con- 
nections of  which  are  shown  in  Fig.  73.  a  and  b  are  the 
main  leads;  c  is  a  lead  to  the  common  terminal  of  the 
motor  two-phase  winding.  R  is  a  resistance  and  L  a  chok- 
ing coil.  The  current  passing  through  R  will  differ  in 
phase  from  that  flowing  through  Z,,  and  the  motor  will 
start,  when  the  switch  is  thrown,  with  a  torque  dependent 
upon  the  phase  difference.  The  maximum  torque  will  be 
developed  when  the  currents  are  90  degrees  apart.  This, 
of  course,  cannot  be  obtained  with  this  device.  By  replac- 
ing the  resistance  by  a  condenser,  a  phase  difference  of 
90  degrees  or  over  can  be  obtained,  with  a  correspondingly 
increased  torque,  and  a  decreased  starting  current.  When 
the  motor  reaches  speed,  the  starting  coils  are  cut  out,  and 
it  then  runs  as  a  single-phase  motor. 


120 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


The  usual  form  of  motor  is  provided  with  a  starting 
device  that  gives  half-load  torque  at  about  150  per  cent  of 
full-load  current.  Full-load  torque  may  be  obtained  at 
somewhat  over  twice  full-load  current,  by  a  special  start- 
ing device. 

In  the  single-phase  induction  motors  produced  by  one 
of  the  largest  American  manufacturers,  a  special  form  of 
automatic  clutch  pulley  is  used.  With  this  arrangement 


Fig.  73. 


the  driving  power  is  not  applied  to  the  pulley  until  the 
armature  has  reached  nearly  full  speed.  Motors  of  this 
type  give  about  one  and  one-half  times  full-load  torque  at 
starting  with  an  input  of  twice  full-load  current.  The 
starting  device  is  a  combined  resistance-reactance,  working 
on  the  principle  already  described. 

The  advantage  of  the  single-phase  induction  motor  over 
the  single-phase  synchronous  motor  lies  principally  in  the 
fact  that  the  latter  motor  is  liable  to  be  thrown  out  of  step 
by  any  fluctuation  in  the  generator  speed.  The  synchro- 
nous motor  is  fairly  efficient  and  can  be  adjusted  to  unity 


INDUCTION   MOTORS.  121 

power  factor,  but  the  current  at  starting  is  very  large  in 
proportion  to  the  feeble  torque. 

A  three-phase  induction  motor  will  give  about  40  per 
cent  of  its  output  when  used  single-phase.  A  two-phase 
motor  will  give  50  per  cent  of  its  two-phase  rating  under 
the  same  conditions.  The  same  motors  can  be  rewound  as 
single-phase  motors,  and  will  then  have  an  output  of  over 
75  per  cent  of  their  former  rating.  The  unaltered  two- 
phase  and  three-phase  motors  can,  however,  be  made  to 
yield,  on  a  single-phase  circuit,  about  75  per  cent  of  their 
rating  by  increasing  the  voltage  30  per  cent  above  that  for 
which  they  are  wound. 

In  the  single-phase  motor  manufactured  by  the  Wagner 
Electric  Company  the  armature  contains  a  definite  winding 
connected  to  a  commutator.  In  starting,  the  armature 
and  field  are  connected  as  in  a  repulsion  motor,  the  princi- 
ples of  which  are  explained  in  the  following  section.  On 
attaining  full  speed  a  centrifugal  device  within  the  arma- 
ture short  circuits  the  commutator  bars  and  the  armature, 
and  the  field  remains  connected  across  the  line.  The  motor 
then  operates  as  a  simple  induction  motor.  No  external 
starting  device  is  required,  there  being  only  two  wires  from 
the  mains  to  the  motor. 

Variable  Speed,  Single-Phase  Motors.  —  As  has  been  seen, 
most  alternating-current  motors  are  inherently  constant 
speed  machines.  The  synchronous  motor  is  strictly  of  this 
description,  while  the  induction  motor  cannot  be  made  to 
operate  at  variable  speed  without  considerable  complication 
or  wasteful  rheostatic  losses. 

Motors  of  the  constant  speed  type  are  eminently  unsuited 
to  certain  classes  of  service,  the  conditions  of  railway  work 
being  a  conspicuous  example.  While  several  installations 


122     POLYPHASE  APPARATUS  AND  SYSTEMS. 

using  the  three-phase  induction  motor  for  heavy  railway 
service  have  been  made  in  Europe,  American  engineers 
do  not  consider  the  induction  motor  as  adapted  to  the 
average  conditions  of  railway  work.  Its  speed  character- 
istics result  in  a  poor  efficiency  of  acceleration  and  in  an 
exaggeration  of  the  station  load  fluctuations  due  to  grades. 
With  the  liberal  air  gap  which  mechanical  considerations 
require  in  a  railway  motor,  the  power  factor  and  the 
apparent  efficiency  are  unsatisfactory.  Furthermore,  the 
use  of  this  type  of  motor  requires  at  least  two  overhead 
conductors,  a  complication  which  has  increased  the  oppo- 
sition to  any  polyphase  system  for  the  working  circuit. 

The  appreciation  of  the  obvious  advantages  to  be  gained 
in  certain  classes  of  railway  work,  if  the  alternating  current 
could  be  used  directly  on  the  trolley  line  without  the  neces- 
sity of  a  reconversion  into  direct  current,  have,  nevertheless, 
foreshadowed  the  development  of  a  satisfactory  form  of 
alternating-current  motor,  which  should,  first,  have  essen- 
tially the  speed-torque  characteristics  of  the  familiar  direct- 
current  seiies  motor,  and  which  should,  second,  be  adapted 
to  single-phase  working  so  as  to  avoid  the  complication  of 
more  than  one  overhead  conductor.  Motors  of  these 
characteristics  have  now  been  successfully  developed,  and 
in  a  variety  of  types  are  already  in  satisfactory  commercial 
service  in  this  country  and  in  Europe.  While  differing  to 
some  extent  in  the  connections  and  arrangement  of  the 
field  circuit,  all  these  types  use  in  common  a  form  of 
armature  not  essentially  different  from  that  of  a  direct- 
current  motor,  the  armature  having  a  commutator  of  usual 
form,  on  which  bear  two  or  more  sets  of  brushes. 

The  types  of  motor  having  the  desired  characteristics 
and  available  for  possible  development  are,  broadly,  the 


INDUCTION    MOTORS. 


123 


simple  series  alternating,  the  compensated  series,  the 
Thomson  repulsion,  and  the  compensated  repulsion  of 
Winter- Eichberg  and  of  Latour. 

In  the  first  of  these,  which  is  the  simplest  in  theory,  we 
have  the  ordinary  series  motor  as  used  in  direct-current 
practice,  with  a  slight  change  in  the  proportions  of  the 
strength  of  the  armature  and  field,  the  entire  magnetic 
circuit  being  laminated  to  reduce  the  eddy  current  loss. 
The  connections  of  this  type  are  shown  diagrammatically 
in  Fig.  74,  in  which  F  and  A  are  respectively  the  field  and 


Fig.  74. 

armature.  The  arrows  show  the  direction  of  the  flux 
generated  by  the  ampere  turns  on  each  member.  The 
motor  terminals  are  at  i  and  2,  the  diagram,  like  those 
following,  being  drawn  for  a  bipolar  motor  and  showing  for 
simplicity  but  one-half  of  the  field  circuit.  As  is  well 
known,  the  series  motor  will  operate  with  alternating 
current  so  far  as  giving  torque  and  speed  is  concerned,  but 
the  alternations  of  the  field  flux  produce  by  transformer 
action  a  voltage  in  that  coil  which  is  short  circuited  by  the 
brush  during  the  instant  of  commutation,  and  this  phenom- 
enon produces  bad  commutation;  also,  the  alternating  flux 


I24 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


due  to  the  current  in  the  armature  induces  a  considerable 
voltage  in  the  armature,  which  is  a  wattless  voltage,  being 
the  E.M.F.  of  self  induction  and  hence  in  quadrature  with 
the  current.  These  two  characteristics  preclude  the  best 
results  from  this  type,  which  is  therefore  mainly  of  theo- 
retical interest. 

The  difference  between  this  motor  and  the  series  com- 
pensated  (Fig.  75)  consists  in  the  important  addition  of 


Fig.  75. 

an  auxiliary  field  winding,  Fv  which  produces  a  magneto- 
motive force  in  the  same  axis  as  that  of  the  armature 
reaction  but  in  the  opposite  direction,  as  indicated  by  the 
arrows.  This  completely  overcomes  the  effect  of  the  alter- 
nating current  in  the  armature  so  far  as  concerns  the  wattless 
voltage  referred  to.  It  also  allows  the  relative  armature 
strength  to  be  greatly  increased  without  inviting  the  poor 
commutation  which  would  result  in  any  machine  having 
too  great  an  armature  strength.  By  the  use  of  an  arma- 
ture of  this  great  relative  strength  it  is  possible  to  attain 


INDUCTION    MOTORS.  125 

satisfactory  torque  with  a  comparatively  weak  field,  torque 
being  proportional  to  the  product  of  the  field  strength  and 
the  armature  strength.  The  advantage  of  this  is  that  the 
field  flux  may  be  made  so  small  that  the  voltage  induced  in 
the  short-circuited  coil  undergoing  commutation  is  reduced 
to  a  value  which  produces  no  harmful  effects  on  commuta- 
tion. This  machine  has  all  the  characteristics  of  the 
direct-current  series  motor,  i.e.,  a  very  large  torque  at 
starting,  and  a  decreasing  torque  with  increasing  speed, 
with  the  possibility  of  operating  at  any  speed  which  is 
allowed  by  the  mechanical  construction  of  the  machine. 

This  motor  takes  its  name  from  the  effect  produced  by 
the  auxiliary  field  winding  referred  to,  which  compensates 
for  the  armature  reaction.  The  compensating  winding 
is  embedded  in  slots  cut  in  the  faces  of  the  main  field  poles, 
and  so  arranged  that  the  direction  of  the  current  in  the 
compensating  winding  at  any  instant  is  opposite  to  the 
direction  of  the  flow  of  current  in  the  armature  conductors 
lying  directly  beneath  it. 

In  the  repulsion  motor,  Fig.  76,  the  field  has  a  distributed 
winding  similar  to  that  of  an  induction  motor  primary. 
The  field  and  armature  circuits  are  not  electrically  con- 
nected, the  current  in  the  armature  being  that  induced  by 
the  stator  or  field  acting  like  the  primary  of  a  transformer. 
The  two  sets  of  brushes  as  shown  are  connected  to  each 
other  by  a  conductor  of  negligible  resistance,  and  the  brushes 
are  given  a  slight  shift  in  the  direction  of  rotation.  It  is 
in  fact  this  shifting  of  the  brushes  that  produces  rotation, 
and  the  direction  of  rotation  depends  on  the  direction  of 
brush  shift.  Thus  the  armature,  short  circuited  through 
the  low  resistance  lead  which  joins  the  opposite  brushes, 
plays  the  part  of  the  short-circuited  secondary  coil  of  a 


126 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


transformer,  but  with  this  difference,  viz.,  that  the  axis  of 
the  flux  produced  by  the  armature  ampere  turns  takes  up  a 
direction  in  space  parallel  to  the  diameter  which  joins  the 
brushes,  whatever  may  be  the  angle  which  this  makes  with 
the  axis  of  the  field  flux. 

If  the  brushes  are  set  on  a  diameter  parallel  with  the  field 
flux  as  at  xy,  a  consideration  of  the  transformer  action 
taking  place  will  show  that  maximum  E.M.F.  is  induced 
since  each  set  of  brushes  is  in  effect  the  terminal  of  a 
secondary  of  many  turns,  all  of  which  are  in  series  and 
which  cut  the  flux  in  such  a  way  that  the  E.M.F  generated 


Fig.  76. 

in  all  the  turns  is  additive.  Maximum  current  will  there- 
fore flow  through  the  armature  windings,  since  the  terminals 
are  short  circuited  on  each  other.  Considering  the  arma- 
ture to  be  equally  divided  by  the  vertical  line  MN,  it  is 
apparent  that  with  the  brushes  at  xy  the  ampere  turns 
produced  by  the  section  MX  are  equal  and  opposite  to  those 
produced  by  the  section  NX.  A  similar  condition  prevails 
in  respect  to  My  and  Ny.  Hence  a  condition  of  no  torque 
results,  the  only  effect  of  the  induced  armature  current 
being  a  force  parallel  to  the  line  of  the  field  flux,  like  the 
repelling  action  existing  between  primary  and  secondary 
coils  in  an  ordinary  transformer.  To  produce  torque  MX 


INDUCTION    MOTORS. 


must  be  unequal  to  NX,  —  in  other  words  there  must  be 
some  resultant  flow  of  current  from  M  around  to  N  or 
vice  versa,  a  condition  which  does  not  exist  under  the 
conditions  assumed. 

Let  the  brushes  now  be  considered  as  placed  on  the 
diameter  MN  perpendicular  to  the  axis  of  field  flux. 
Since  the  direction  of  field  flux  is  such  as  to  produce  maxi- 
mum armature  E.M.F.  between  the  points  xy,  the  differ- 
ence of  potential  between  M  and  N  is  zero,  and  no  current 
will  flow  in  the  armature  windings. 

We  have,  therefore,  the  two  extreme  positions  in  which 
the  brushes  may  be  placed.  In  the  first  we  have  maximum 
armature  current  but  no  torque,  and  in  the  second,  zero 
armature  current  and  also  zero  torque.  Since  all  the 
current  flowing  in  the  armature  is  an  induced  current, 
requiring  that  some  E.M.F.  shall  exist  between  the  short- 
circuited  brushes,  it  follows,  first,  that  the  brushes  shall 
be  set  at  some  point  away  from  MN  so  that  an  E.M.F. 
may  be  set  up,  and,  second,  that  they  shall  be  set  at  some 
point  away  from  xy  so  that  the  current  thus  induced  shall 
have  a  resultant  value  in  the  direction  MN  so  as  to  pro- 
duce torque.  Since  the  amount  of  current  that  flows  in  the 
armature  varies  with  the  amount  by  which  the  brushes  are 
angularly  displaced  from  MN,  and  since  the  proportion  of 
this  induced  current  that  is  effective  in  producing  torque 
varies  with  the  angular  displacement  from  xy,  there  will 
be  some  intermediate  position  corresponding  to  maximum 
torque.  In  the  average  motor  this  position  is  along  the 
diameter  ab  where  the  angle  $  is  about  15  degrees.  By 
altering  the  angle  c/>  it  will  be  seen  that  the  torque  for  a 
given  speed  may  be  raised  or  lowered  at  will.  This  machine 
has  the  same  characteristics  as  the  compensated  series 


128     POLYPHASE  APPARATUS  AND  SYSTEMS.. 

motor  as  to  decrease  of  torque  with  increase  of  speed,  and 
at  the  instant  of  starting  its  performance  is  identical  with 
that  of  the  compensated  series  machine.  At  a  speed  near 
synchronism  the  resultant  field  due  to  the  flux  set  up  by 
the  primary  and  that  due  to  the  flux  produced  by  the 
armature  current  takes  on  the  character  of  the  revolving 
field  of  a  polyphase  motor,  and  there  is  thus  no  pulsation 
of  flux  through  the  coil  that  is  spanned  by  the  brush  during 
the  instant  of  commutation.  Near  synchronism,  therefore, 
the  commutation  is  better  than  in  the  series  or  the  com- 
pensated series  type;  but  as  the  speed  increases  above  this 
point  the  field  loses  its  rotary  character  and  the  field  due 
to  primary  flux  predominates.  This  causes  a  considerable 
voltage  to  be  generated  by  rotation  in  the  coil  undergoing 
commutation,  which  produces  bad  commutation  as  the 
speed  increases,  so  that  the  practical  limit  of  speed  is  some- 
where below  one  and  one-half  times  synchronous  speed. 
Thus  in  a  four-pole,  25  cycle  motor  very  good  commutation 
would  be  expected  at  750  revolutions,  but  the  maximum 
tolerable  speed  would  be  limited  to  about  1000  revolutions. 
It  has  been  seen  that  in  the  repulsion  motor  the  action 
taking  place  in  the  armature  is  that  due  to  the  two  com- 
ponents acting  at  right  angles  to  each  other  into  which  the 
induced  armature  current  may  be  considered  as  resolved. 
Instead  of  effecting  this  resolution  by  means  of  brush  shift 
it  is  apparent  that  the  equivalent  result  may  be  produced  by 
introducing  into  the  armature  two  separate  currents  at 
right  angles,  selecting  for  each  such  a  value  that  their  resul- 
tant shall  produce  an  effect  similar  to  the  single  current 
which  exists  in  the  armature  of  the  repulsion  motor.  With 
proper  relative  values  for  these  two  separate  currents  it 
will  be  seen  that  their  resultant  can  be  made  to  act  along  an 


INDUCTION    MOTORS. 


I29 


axis  which  makes  an  angle  <£  of  1 5  degrees  with  that  of  the 
field  flux,  and  that  a  variation  in  the  relative  strengths  of 
the  two  currents  will  effect  any  desired  change  in  the 
magnitude  of  the  angle  <f>.-  On  this  principle  are  devised 
the  so-called  compensated  repulsion  motors  of  Winter- 
Eichberg  and  of  Latour,  which  are  similar  to  each  other  in 
theory,  but  different  slightly  in  the  brush  arrangement.  It 
will  therefore  be  sufficient  to  describe  the  Winter- Eichberg 
type,  of  which  the  connections  are  given  in  Fig.  77. 

The  current  from  the  primary  or  stator  is  led  into  the 
rotor  by  the  brushes  W  at  right  angles  to  the  axis  of  main 


Fig.  77. 


action  aaf.  It  is  this  current  which  generates  the  torque- 
producing  field,  the  main  stator  winding  being  regarded 
merely  as  the  primary  of  a  transformer  which  induces 
secondary  currents  in  the  armature  in  the  direction  afa. 
The  resultant  of  the  two  actions  along  the  diameters  b'b 
and  a' a  is  effective  in  the  direction  of  the  full-line  arrow 
within  the  circle.  The  angle  of  resultant  action  may  be 
varied  by  changing  the  relative  magnitude  of  the  com- 
ponent actions  as  explained.  This  is  effected  in  practice  by 
supplying  the  circuit  W  from  the  secondary  of  a  current 
transformer,  not  shown,  instead  of  passing  the  main  current 


130     POLYPHASE  APPARATUS  AND  SYSTEMS. 

directly  into  bbf.  Taps  in  the  secondary  winding  of  the 
current  transformer  permit  any  desired  value  to  be  given 
to  the  component  bb',  a  regulation  being  thus  secured 
which  is  similar  to  that  ensuing  when  a  change  is  made  in 
the  angle  of  brush  shift  in  the  plain  repulsion  motor. 

Motors  of  all  these  types  operate  best  on  circuits  of  low 
frequency,  and  in  most  installations  the  current  supply  is 
at  25  cycles.  For  very  large  motors  the  use  of  still  lower 


Fig.  78. 

frequency  makes  it  easier  to  secure  a  good  design,  and  it 
has  in  some  cases  been  proposed  to  go  as  low  as  12.5  cycles. 
In  fact  the  lower  the  frequency  the  better  will  be  the  possible 
performance  and  the  less  the  cost  of  a  motor  of  given 
capacity,  a  condition  which  reaches  its  obvious  limit  with  a 
current  supply  at  zero  frequency,  in  other  words,  with  direct 
current.  To  secure  satisfactory  commutation,  motors  like 
the  plain  series  or  the  series  compensated,  in  which  the 


INDUCTION    MOTORS.  13! 

main  current  passes  directly  into  the  armature,  are  best 
designed  for  a  low  potential  not  exceeding,  say,  300  volts. 
In  the  repulsion  and  the  Winter- Eichberg  types  where  the 
field  circuit  is  not  in  electrical  connection  with  the  armature 
it  is  theoretically  possible  to  wind  the  stator  for  very  high 
potentials  and  thus,  in  the  case  of  a  high  voltage  trolley, 
dispense  with  the  step-down  transformer  which  with  the 
other  types  of  motor  must  be  carried  on  the  car.  Experience 
has  demonstrated,  however,  that  a  high  voltage  winding, 


Fig.  79. 

even  in  the  stationary  element,  cannot  safely  be  employed 
in  a  machine  like  a  railway  motor,  which  must  be  of  the 
most  rugged  construction  and  necessarily  subject  to  less 
care  than  can  be  given  to  a  stationary  motor,  and  in  which, 
finally,  the  space  at  the  designer's  disposal  is  strictly  limited 
by  considerations  of  track  gauge,  wheel  base,  and  wheel 
diameter.  For  this  reason  these  types  are  now  designed  for 
about  750  volts  as  a  maximum. 

The   general   appearance   of   the   field   structure   of  all 
these  motors  is  illustrated  in   Fig.    78,   which  shows  the 


132     POLYPHASE  APPARATUS  AND  SYSTEMS. 

cylindrical  laminated  body  carrying  the  field  winding  which 
is  embedded  in  slots  cut  in  the  interior  of  the  ring.  The 
general  mechanical  design  follows  the  lines  established  by 
direct-current  practice,  as  will  be  seen  from  Fig.  79,  show- 
ing a  completed  motor  housed  in  its  supporting  and  enclos- 
ing frame. 


SYNCHRONOUS    MOTORS.  1 33 


CHAPTER     VI. 
SYNCHRONOUS    MOTORS. 

General.  —  Any  alternating-current  generator,  with  little 
or  no  change,  can  be  used  as  a  synchronous  motor.  Elec- 
trically and  mechanically  the  motor  resembles  the  corre- 
sponding generator,  and  must  be  provided  with  the  same 
station  equipment,  including  some  source  of  exciting  cur- 
rent. The  synchronous  motor,  especially  in  units  of  large 
output,  possesses  a  number  of  features  which  makes  its 
use  at  times  preferable  to  that  of  the  induction  motor. 
Besides  the  advantage  of  an  unvarying  speed  at  all  loads, 
the  power  factor  can  be  altered  at  will  by  changing  the 
exciting  current  and  made  equal  to  unity  at  any  load. 
The  current  can  even  be  made  leading,  to  offset  a  lagging 
current  in  other  parts  of  the  system.  The  synchronous 
motor,  especially  at  low  speeds,  is  cheaper  to  build  than 
the  induction  motor,  and  its  efficiency,  as  a  rule,  will  be 
found  to  be  higher. 

As  a  partial  offset  to  these  advantages,  the  synchronous 
motor  is  not  adapted  for  use  where  a  large  starting  torque 
or  frequent  starting  of  the  load  is  necessary.  It  does  not 
admit  of  independent  speed  regulation.  It  also  has  the 
disadvantage  of  requiring  certain  station  appliances  and 
an  exciting  current.  Another  objection  to  the  synchro- 
nous motor  is  its  tendency  to  hunt  unless  the  conditions  of 
operation  are  properly  chosen. 


134    POLYPHASE  APPARATUS  AND  SYSTEMS. 

Speed.  —  The  speed  of  the  synchronous  motor  is  not 
necessarily  the  generator  speed,  but  a  speed  which,  mul- 
tiplied by  the  number  of  poles,  gives  a  product  equal  to 
the  generator  alternations.  A  motor,  with  twice  as  many 
poles  as  the  generator,  will  have  half  its  speed,  or  vice 
versa.  As  load  is  thrown  on  the  synchronous  motor, 
there  is  a  lag  in  the  relative  positions  of  armature  winding 
and  pole  face,  corresponding  to  a  change  in  phase  displace- 
ment between  the  impressed  and  the  counter  E.M.F.  The 
effective  counter  E.M.F.  is  thereby  reduced,  which  permits 
a  larger  flow  of  current. 

The  motor  speed  is  independent  of  the  voltage  and  can- 
not be  altered  except  by  changing  the  generator  speed. 
It  is  important,  therefore,  that  the  regulation  of  the  prime 
mover  be  as  perfect  as  possible,  both  in  the  number  of 
revolutions  per  minute  and  in  the  angular  speed;  other- 
wise, as  the  fly-wheel  capacity  of  the  motor  is  sufficient  to 
absorb  considerable  energy,  it  tends  to  run  at  constant 
speed,  and  if  the  source  of  power  is  pulsating,  heavy  fluc- 
tuating currents  are  set  up  between  the  motor  and  gener- 
ator, reducing  the  motor  capacity  and  producing  voltage 
fluctuations  which  may  be  prohibitive. 

Torque  and  Output.  —  A  synchronous  motor  at  starting 
acts  somewhat  as  an  induction  motor.  Consequently,  any 
variation  of  its  proportions,  such  as  the  shape  of  the  pole 
pieces,  armature  reaction,  and  nature  of  the  winding  - 
i.e.,  distributed  or  unitooth  —  affects  its  starting  torque. 
The  starting  torque  may  vary  from  nothing  to  20  or  30 
per  cent  of  full-load  running  torque,  depending  upon  the 
motor  design.  When  once  in  motion,  the  motor  will 
rapidly  attain  synchronous  speed.  After  synchronism  is 
attained,  polyphase  motors,  as  usually  constructed,  will 


SYNCHRONOUS    MOTORS.  135 

carry  four  to  five  times  full  load  unless  the  applied  poten- 
tial is  allowed  to  fall  below  normal.  If  further  loaded, 
they  fall  out  of  synchronism,  and  must  be  started  afresh. 
Single-phase  synchronous  motors  have  dead  points,  and 
will  not  start  from  rest,  some  extraneous  source  of  power 
being  necessary  to  start  and  to  bring  up  to  speed  motors 
of  this  type.  This  is  usually  effected  by  an  induction  motor. 
In  some  cases  where  a  direct-current  source  of  power  is 
available  the  direct-connected  (or  belted)  exciter  may  be 
used  as  a  starting  motor,  although  the  exciter,  unless  spe- 
cially proportioned  for  use  in  this  way,  will  usually  be  of 
insufficient  capacity.  The  starting  effort  necessary  to  bring 
a  synchronous  motor  up  to  speed  may  be  large,  sometimes 
as  great  as  20  per  cent  of  the  rated  full-load  torque  of  the 
motor,  and  this  maximum  condition  is  usually  beyond  the 
torque  that  can  be  delivered  by  the  exciter,  whose  rating  is 
seldom  higher  than  five  per  cent  of  the  motor  rating. 

The  limit  to  the  torque  and  output  of  a  synchronous 
motor  is  dependent  mainly  upon  the  terminal  voltage. 
Under  rated  voltage  the  margin  of  most  motors,  before 
the  break-down  point  is  reached,  is  sufficient  to  enable 
them  to  stand  a  heavy  overload.  Variation  of  the  speed 
of  the  prime  mover  will  reduce  the  maximum  output. 

Voltage.  —  -  The  relation  of  impressed  volts  to  the  max- 
imum output  is  the  same  in  synchronous  as  in  induction 
motors,  the  output  and  the  starting  torque  varying  within 
certain  limits  as  the  square  of  the  volts.  It  is  essential, 
therefore,  that  the  pressure  be  kept  at  the  rated  voltage  of 
the  motor;  otherwise  the  motor  may  not  start  at  all,  par- 
ticularly if  it  consumes  an  excessive  starting  current. 

Synchronous  motors  may  be  wound  for  a  voltage  as 
high  as  that  of  alternating-current  generators  of  equal 


136     POLYPHASE  APPARATUS  AND  SYSTEMS. 

capacity  and  like  construction.  Motors  of  the  revolving 
armature  type  are  not  therefore  well  adapted  to  very  high 
potentials,  and  have  been  almost  completely  superseded 
by  the  revolving  field  type  by  reason  of  the  same  advan- 
tages as  are  secured  thereby  in  the  construction  of  gener- 
ators. A  further  advantage  is  that  motors  of  the  revolving 
field  type  as  ordinarily  proportioned  have  a  somewhat 
greater  starting  torque  than  those  of  the  revolving  arma- 
ture type  on  account  of  the  greater  arc  covered  by  the 
pole  face.  Motors  of  100  kilowatts  to  500  kilowatts  are 
frequently  wound  for  potentials  as  high  as  6000  volts; 
larger  motors  are  frequently  designed  for  12,000  or  13,000 
volts,  and  these  limits  can  be  exceeded  by  several  thousand 
volts  if  occasion  demands,  although,  as  in  the  case  of  gen- 
erators, it  is  usually  found  more  convenient  and  less  costly 
to  interpose  transformers  than  to  design  the  windings  for 
potentials  much  in  excess  of  about  13,000  volts. 

Methods  of  Starting.  —  When  a  large  torque  is  required 
to  turn  over  the  load,  as  in  the  case  of  mill  machinery  or 
long  lines  of  shafting  and  belting,  a  friction  clutch  must 
be  used.  This  permits  the  load  to  be  gradually  thrown  on 
the  motor  after  it  reaches  synchronism.  The  clutch  may 
be  mounted  on  the  motor  base  extended,  an  extra  standard 
being  required  for  this  purpose;  or  the  motor  may  be 
belted  to  a  line  shaft  on  which  there  is  a  coupling.  This 
is  the  cheaper  and  more  usual  method.  Fig.  80  shows  a 
300-H.P.  motor,  built  with  extended  base,  carrying  a  clutch 
and  driving  pulley.  In  selecting  a  coupling  for  this  class 
of  work,  one  of  ample  proportions  should  be  used,  as  it 
must  start  the  load  gradually,  without  exceeding  the  break- 
down point  of  the  motor,  and  without  overheating. 

The   operations   in   starting   a    synchronous   motor   are 


SYNCHRONOUS    MOTORS. 


138     POLYPHASE  APPARATUS  AND  SYSTEMS. 

about  as  follows:  First,  the  main  switch  is  closed  and  the 
motor  with  its  fields  tmexcited  will  start  with  a  small 
torque  due  to  the  induced  currents  in  the  pole  pieces,  and 
gradually  acquire  speed.  When  maximum  speed  is  at- 
tained, —  and  this  may  be  something  less  than  synchronous 
speed,  —  the  current  from  the  exciter  can  be  switched  into 
the  fields,  the  exciter  commonly  being  direct-connected 
to  or  belt-driven  by  the  motor,  or  in  the  case  of  a  single 
station  containing  several  synchronous  motors,  being 
driven  from  a  separate  source  of  power  and  of  sufficient 
capacity  to  excite  all  motors  simultaneously.  At  the 
moment  considered,  when  the  motor  has  nearly  reached 
synchronous  speed,  the  angular  velocity  of  the  field 
structure  (considering  the  stationary  armature  type  of 
machine)  is  nearly  equal  to  that  of  the  rotating  induced 
field  engendered  by  the  armature  current,  so  that  as  soon 
as  the  field  winding  is  excited  the  rotating  structure  will 
accelerate  sharply  and  "lock  into  step."  The  full  load 
can  then  safely  be  thrown  on  the  motor  by  the  friction 
clutch,  if  one  is  used. 

With  full  voltage  impressed  the  current  taken  at  starting 
may  be  anything  from  150  per  cent  of  full-load  current  to 
several  times  normal  current,  being  limited  by  the  resis- 
tance and  self-induction  of  the  armature  winding,  i.e.,  its 
impedance.  This  excessive  starting  current,  as  it  is  of 
an  inductive  character,  may  cause  a  large  drop  in  the 
line,  and  disarrange  the  voltage  of  the  entire  system. 

If  the  motor  takes  a  large  proportion  of  the  generator 
output,  or  is  used  in  connection  with  lights,  and  started 
and  stopped  at  frequent  intervals,  some  means  should  be 
employed  to  reduce  the  current.  This  can  be  done,  as  in 
the  case  of  the  induction  motor,  by  the  use  of  a  resistance, 


SYNCHRONOUS  MOTORS. 


139 


a  reactance,  or  a  compensator  in  the  main  circuit,  or  by  a 
small  starting  motor.  A  compensator  starter  is  often  used. 
This  may  be  of  substantially  the  type  illustrated  for  use 
with  induction  motors  (see  Figs.  56  and  57)  or  may  be  of 
rather  more  elaborate  construction  with  a  large  number  of 
steps  like  that  shown  in  Fig.  81. 

With  50  per  cent  of  the  impressed  volts,  the  synchronous 
motor,  when  properly  proportioned,  will  take,  at  starting, 


Fig.  81. 

a  current  from  the  line  equal  to  about  full-load  current, 
and  start  with  a  torque  about  15  per  cent  of  the  full-load 
running  torque.  Where  the  magnitude  of  the  starting  cur- 
rent is  of  no  consideration  and  the  highest  possible  starting 
torque  is  essential,  the  compensator  may  be  designed  to 
raise  instead  of  to  lower  the  line  potential.  In  this  way 
with  a  properly  designed  motor,  a  starting  effort  equal  to 
two-thirds  normal  full-load  torque  may  be  produced. 


140    POLYPHASE  APPARATUS  AND  SYSTEMS. 

This,  however,  represents  an  unusual  condition,  and  such 
arrangements  are  adopted  only  in  rare  instances. 

A  starting  motor,  of  the  induction  type,  is  the  best 
means  of  reducing  the  current  at  starting.  The  current 
taken  by  this  small  motor  cannot  seriously  affect  the  volt- 
age of  the  circuit.  This  method  should  be  employed  when 
the  motor  is  started  frequently,  or  when  a  low  starting  cur- 
rent is  essential  to  preserve  good  regulation.  Where  no 
load  is  connected  to  the  motor  at  the  time  of  starting,  a 
starting  motor  having  a  nominal  rating  of  about  one- 
eighth  the  capacity  of  the  synchronous  motor  will  usu- 
ally be  found  of  sufficient  size  to  meet  all  average  condi- 
tions. Such  a  motor  should  be  capable  of  a  maximum 
torque  equal  to  about  double  its  own  rating,  or,  say,  equal 
to  one-quarter  of  the  normal  full-load  torque  of  the  syn- 
chronous motor.  The  continuous  output  rating  of  a 
starting  motor  is  a  matter  of  small  consequence,  the  only 
factor  of  importance  being  its  ability  to  deliver  high  torque 
for  the  short  period  of  time  necessary  in  getting  up  to 
speed.  With  a  well-designed  starting  motor,  proportioned 
to  give  a  maximum  torque  per  unit  of  current  input,  syn- 
chronous motors  without  load  can  be  started  with  a  cur- 
rent from  the  line  not  exceeding  about  one-third  their 
normal  full-load  rating.  Such  starting  motors,  to  secure 
good  torque,  have  high  resistance  armatures  and  hence 
large  slip.  When  direct-connected  they  are  designed  with 
a  fewer  number  of  poles  than  the  synchronous  motor, 
so  that  their  synchronous  speed  minus  the  slip,  in  other 
words,  their  free  running  speed,  shall  be  such  as  to  drive 
the  synchronous  motor  at  a  speed  slightly  in  excess  of 
normal.  The  equivalent  result  where  the  starting  motor 
is  geared  is  secured  by  choosing  a  suitable  gear  ratio.  In 


SYNCHRONOUS    MOTORS.  141 

this  way  when  the  attained  speed  of  the  motor  is  brought 
above  normal,  current  is  cut  off  from  the  induction  motor 
and  the  synchronous  motor  is  switched  in  at  the  moment 
synchronism  is  reached  during  the  gradual  fall  in  speed 
that  follows  the  removal  of  the  driving  power.  Where  the 
maximum  speed  is  only  slightly  in  excess  of  the  normal,  a 
fine  adjustment  is  possible  by  somewhat  increasing  the 
excitation  of  the  synchronous  motor.  This  increases  the 
core  loss  of  the  synchronous  motor  and  thus  the  power 
required  to  drive  it,  and  the  high-slip  induction  motor 
responding  sensitively  in  speed  change  to  variation  of  load 
will  drop  to  a  speed  at  which  the  necessary  synchronizing 
may  be  effected. 

Where  the  synchronous  motor  has  a  small  number  of 
poles  it  may  be  difficult  to  design  a  direct-connected  start- 
ing motor  with  enough  slip  to  bring  its  free  running  speed 
down  to  that  of  the  synchronous  motor.  In  such  cases  a 
resistance  intercalated  in  the  circuit  supplying  the  starting 
motor,  this  resistance  being  connected  in  after  full  speed 
is  reached,  will  produce  the  desired  result,  whereupon  syn- 
chronizing may  be  effected  in  the  manner  just  described. 

Synchronizing  of  motors  with  each  other  or  with  the 
supply  system  is  effected  in  precisely  the  same  manner  as 
in  the  case  of  generators.  As  in  the  case  of  induction 
motors,  reversal  of  the  direction  of  rotation  is  secured  by 
reversal  of  any  two  leads  in  a  three-phase  motor  and  by 
reversing  the  leads  of  either  phase  in  a  two-phase  motor. 

Fig.  82  showsa6oo-H.P.  5o-cycle  three-phase  motor  with 
direct-connected  induction  motor  starter.  The  synchro- 
nous motor  has  30  poles  and  operates  at  200  revolutions 
per  minute.  The  induction  starter  has  24  poles  and  is 
given  a  nominal  rating  of  75  H.P. 


142     POLYPHASE  APPARATUS  AND  SYSTEMS. 


HraHndi 


SYNCHRONOUS    MOTORS.  143 

Fig.  83  shows  a  225-H.P.  motor  with  geared  starting 
motor.  The  lever  shown  at  the  left  slides  the  motor  pinion 
out  of  contact  with  the  large  gear  wheel  as  soon  as  the 
synchronous  motor  has  been  switched  on  the  line.  This 
motor  is  direct-connected  to  an  air  compressor  at  a 
mining  plant  in  Mexico. 

Many  forms  of  self-starting  synchronous  motors  have 
been  devised  for  use  on  single-phase  circuits.  Most  of 
these  are  provided  with  a  commutator  for  self-excitation, 
and  a  starting  device.  A  commutator,  in  series  with  the 
field  winding,  rectifies  the  current  at  the  instant  the  main 
armature  current  is  in  phase  for  producing  a  slight  torque. 
When  the  motor  reaches  speed,  the  commutator  is  cut  out. 
One  of  these  types  is  the  single-phase  motor, made  by  the 
Fort  Wayne  Company,  which  embodies  a  modification  of 
this  construction.  The  main  current  is  first  thrown  on  a 
continuous-current  winding  connected  to  a  commutator, 
and  laid  over  the  alternating-current  winding  on  the  arma- 
ture, which  is  connected  to  collector  rings*  When  the 
motor  reaches  synchronism,  the  main  current  is  switched 
into  the  alternating-current  winding,  and  the  field  circuit 
closed  on  the  starting  winding  through  the  commutator. 

When  a  synchronous  motor  is  started  by  the  application 
of  voltage  to  its  armature  terminals,  the  armature  wind- 
ing plays  the  part  of  a  transformer  primary  with  respect  to 
the  field  winding,  which  latter  acts  as  the  secondary.  If 
the  ratio  of  field  turns  to  armature  turns  is  large,  high 
voltages  may  consequently  be  induced  in  the  field  circuit. 
These  are  reduced  as  far  as  possible  by  using  a  minimum 
number  of  turns  in  the  field,  designing  it,  in  other  words,  for 
a  moderate  excitation  potential,  as  low  as  50  volts  in  ex- 
treme cases.  In  the  revolving  field  type,  where  the  use  of 


144     POLYPHASE  APPARATUS  AND  SYSTEMS, 


s 


SYNCHRONOUS    MOTORS. 


145 


strip  copper  for  the  exciting  winding  gives  a  small  number 
of  turns  and  makes  it  easy  to  insulate  the  coil  thoroughly, 
these  induced  potentials  are  kept  within  moderate  limits 
so  that  no  danger  to  the  machine  results.  The  voltages 
generated  in  this  way,  nevertheless,  are  frequently  high 
enough  to  give  a  dangerous  shock,  and  operators  are  there- 
fore careful  to  avoid  contact  with  any  part  of  the  field 
circuit  during  starting. 


Fig   84. 

In  the  case  of  stationary  field  machines,  including  rotary 
converters,  it  is  customary  to  cut  down  the  magnitude  of 
the  induced  potential  by  breaking  up  the  fields  into  a 
number  of  parts,  or  by  open-circuiting  each  field  spool,  as 
shown  in  Fig.  84.  Leads  from  each  spool  are  brought  out 
to  convenient  switches  on  the  motor  frame.  The  motor  is 
started  with  these  open.  When  synchronism  is  reached 
the  switches  are  closed,  thus  putting  the  field  coils  in 
series,  and  throwing  them  in  circuit  with  the  exciter. 

Where  a  synchronous  motor  is  so  connected  as  to  take 
the  entire  output  of  a  generator,  it  is  possible  to  start  both 


146  POLYPHASE   APPARATUS   AJtfU.  SYSTEMS. 

motor  and  generator  from  rest  simultaneously.  The  pro- 
cedure is  then  as  follows.  All  switches  between  the  two 
are  closed,  and  the  generator,  strongly  excited,  is  slowly 
revolved.  The  motor,  with  its  field,  circuit  not  yet  ex- 
cited, now  starts  up,  revolving  at.  a  speed  corresponding  to 
the  low  frequency  which  is  being  supplied  to  it. 

As  soon  as  the  motor  is  well  in1  motion  it  is  given  full 
excitation  and  locks  into  step  with  the  generator.  'The 
speed  of  the  latter  is  gradually  .increased,  the  speed  of  the 
motor  accelerating  simultaneously,  till  at  last  full  speed  is 
reached  and  normal  conditions  of  operation  attained. 

Motors  with  "  Amortisseur  "  Winding.  —  Synchronous 
motors  are  frequently  equipped  with  an  auxiliary  winding 
consisting  of  brass  or  copper  bars  imbedded  in  the  pole 
face  and  having  their  ends  connected  by  short-circuiting 
rings.  Such  an  arrangement  is  termed  an  "  amortisseur " 
winding,  and  amounts  in  effect  to  superposing  on  the  field 
structure  a  winding  analogous  to  that  of  the  armature  of  a 
squirrel-cage,  or  short-circuited,  type  induction  motor.  At 
the  moment  of  starting,  therefore,  a  synchronous  motor  so 
equipped  has  the  characteristics  of  a  squirrel-cage  induc- 
tion motor  with  high  resistance^  short-circuited  secondary, 
with  the  result  that  the  starting  torque  is  much  higher  than 
in  synchronous  motors  of  ,  the  ordinary  form.  By  this 
arrangement  it  is  in  most  cases  easy  to  secure,  with  one 
and  one-half  times  full-load  current  m  the  line,  a  starting 
torque  equivalent  to  one-third  of  the  full-load  running 
torque.  The  regulation  of  the  motor,  that  is,  its  sensi- 
tiveness in  respect  to  change  of  power  factor  with  change 
of  load,  is  also  improved  by  reason  of  the  compensating 
action  which  the  squirrel-cage  winding  provides.  A  motor 
of  this  type,  with  constant  field  excitation,  will  therefore 


SYNCHRONOUS   MOTORS.  147 

maintain  a  more  nearly  constant  power  factor  with  changes 
of  load  or  changes  in  the  line  conditions.  The  action  of 
the  squirrel-cage  winding  in  this  respect,  and  with  refer- 
ence to  the  improved  stability  or  freedom  from  hunting 
which  it  gives,  is  similar  to,  but  more  effective  than,  that 
achieved  by  the  use  of  metal  bridges  between  the  pole  tips 
(such  as  are  illustrated  in  Fig.  21).  Eddy  currents  of 
greater  or  less  magnitude  always  exist  in  such  metal  bridges, 
and  as  these  eddy-current  losses  are  less  in  the  squirrel- 
cage  structure,  the  efficiency  of  the  motor  is  correspondingly 
better.  Motors  of  this  type  are  thus  seen  to  possess,  par- 
ticularly in  respect  to  starting,  certain  characteristics  of  an 
induction  motor,  and  are  therefore  sometimes  referred  to 
as  "synchronous  induction  motors." 

Field  Excitation.  —  Since  the  synchronous  motor  runs 
at  constant  speed,  an  increase  in  the  field  excitation  will 
cause  a  corresponding  increase  in  the  counter  E.M.F. 
generated  in  the  motor.  If  the  field  excitation  is  suffi- 
ciently increased,  this  counter  E.M.F.  would  thus  be  made 
considerably  greater  than  the  E.M.F.  impressed  at  the 
motor  terminals.  Since  the  counter  E.M.F.  must  always 
assume  such  a  value  with  relation  to  the  impressed  E.M.F. 
as  will  permit  to  flow  into  the  motor  that  particular  value 
of  current  corresponding  to  the  load,  and,  further,  since 
the  adjustment  of  counter  E.M.F.  necessary  to  fulfill  this 
condition  cannot  be  effected  by  automatic  change  of  speed 
(as  in  the  case  of  a  direct-current  motor),  the  only  way  in 
which  the  counter  E.M.F.  can  alter  will  be  by  a  change  in 
the  flux  which  produces  it.  This,  in  a  synchronous  motor 
over-excited  in  the  way  assumed,  is  brought  about  by 
change  of  phase  of  the  current  with  respect  to  the  E.M.F. 
The  current  will  therefore  advance  in  phase  ahead  of  the 


148    POLYPHASE  APPARATUS  AND  SYSTEMS. 

E.M.F.,  since  in  this  way,  by  armature  reaction,  the  flux 
in  the  armature  generated  by  the  leading  current  will  be 
directly  opposed  to  the  flux  generated  by  the  field  winding. 
The  amount  of  lead  will  be  such  that  the  net  flux,  that  is, 
the  difference  between  the  flux  produced  by  the  field  and 
the  opposing  flux  generated  by  the  leading  current  in  the 
armature,  is  brought  to  a  value  which  will  produce  a 
counter  E.M.F.  proportionate  to  the  load.  Conversely, 
if  a  weak  field  excitation  is  applied,  a  lagging  current 
will  be  taken,  the  armature  flux  produced  by  it  being 
additive  to  that  generated  in  the  field  poles. 

It  follows  then  that  by  simply  changing  the  strength 
of  the  exciting  current,  the  armature  current  for  any  con- 
dition of  load  of  the  synchronous  motor  can  be  made  lag- 
ging or  leading  with  respect  to  the  impressed  E.M.F.  or 
in  phase  with  it.  In  other  words,  the  current  input  to  the 
motor  depends  upon  the  field  excitation,  although  the 
amount  of  energy  consumed  at  a  given  load  remains  un- 
changed except  so  far  as  it  is  affected  by  minor  variations 
in  the  efficiency  under  the  different  conditions  of  operation. 

The  effect  upon  the  armature  current  produced  by 
varying  the  field  excitation  is  shown  by  the  curves  in 
Fig.  85.  The  values  at  the  left  of  the  diagram  are  those 
corresponding  to  a  low  excitation,  and  the  armature  current 
is  lagging.  With  increase  of  excitation,  the  angle  of  lag 
and  the  current  input  diminish  until  a  minimum  current 
input  is  reached,  at  which  the  current  is  in  phase  with 
the  E.M.F.  A  further  increase  in  excitation  increases  the 
current  input,  which  is  now  leading  with  respect  to  the 
E.M.F.,  the  angle  of  lead  and  the  current  input  continuing 
to  increase  as  the  excitation  is  increased  until  saturation 
of  the  -magnetic  circuit  is  reached. 


SYNCHRONOUS    MOTORS. 


149 


There  is,  therefore,  one  value  of  the  exciting  current  for 
any  given  load  for  which  the  armature  current  is  a  mini- 
mum, and  at  which  the  motor  will  be  operating  at  unity 
power  factor.  In  motors  of  good  regulation,  this  value 
varies  but  slightly  with  different  loads,  a  fact  which  is 
especially  noticeable,  for  the  reasons  already  suggested,  in 
the  case  of  motors  equipped  with  squirrel-cage  winding. 
The  term  "phase  characteristic"  is  given  to  the  curves 


Exciting  Current^ 
Fig.  85. 

\ 
showing  the  change  of  armature  current  with  variation  of 

field  excitation.  They  are  sometimes  referred  to  also  as 
Mordey's  "V  curves,"  from  the  name  of  the  English 
engineer  who  was  the  first  to  determine  the  existence  and 
nature  of  the  phenomenon. 

The  result  obtained  from  this  property  of  the  synchro- 
nous motor,  of  producing  at  will  any  displacement  of  phase 
between  current  and  E.M.F.,  is  the  possibility  of  annulling 
the  reactance  due  to  the  inductance  of  the  line,  and  at  the 
same  time  of  compensating  for  a  certain  amount  of  lagging 


150 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


current  due  to  inductive  loads  in  other  parts  of  the  circuit. 
When  over-excited,  the  synchronous  motor  acts  like  a 
great  condenser.  It  will  take  care  of  a  total  current  made 
up  of  energy  and  wattless  components,  to  an  extent  equal 
to  its  rated  ampere  output. 

Synchronous  motors  which  are  used  primarily  to  pro- 
duce leading  current  for  the  purposes  indicated,  are  fre- 
quently called  "rotary  condensers."  When  used  for  this 
purpose  they  may  be  run  light  or  made  to  convert  a  cer- 


n 


L 


Fig.  86. 


1000  Energy  Kw. 

Fig.  87. 


tain  proportion  of  the  current  input  into  useful  mechanical 
work.  This  proportion,  that  is,  the  energy  component  of 
the  current  input,  may  be  made  to  bear  any  desired  rela- 
tion to  the  leading  wattless  component.  If  the  mechan- 
ical load  is  sufficient,  the  electrical  and  magnetic  material 
in  the  synchronous  motor  is  used  to  the  best  advantage 
when  the  arithmetic  sum  of  the  leading  and  energy  com- 
ponents is  a  maximum.  This  condition,  when  the  field  is 
adjusted  to  give  71  per  cent  power  factor  leading,  is  shown 
in  the  diagram  (Fig.  86).  Assuming  that  a  system  has  an 
inductive  (lagging)  load  of  1414  apparent  kilowatts  at 


SYNCHRONOUS    MOTORS.  15! 

71  per  cent  power  factor,  that  is,  1000  actual  kilowatts,  it 
is  evident  from  the  diagram  shown  in  Fig.  87  that  adding 
500  wattless  kilowatts  of  leading  current  will  reduce  the 
load  to  1 1 20  apparent  kilowatts,  or  89  per  cent  power 
factor.  In  this  case  294  (1414  minus  1120)  apparent 
kilowatts  are  gained.  It  will  be  noted,  however,  that  a 
further  addition  of  500  wattless  kilowatts  of  leading  cur- 
rent to  the  system  at  the  higher  power  factor  of  89  per 
cent  would  save  only  120  (1120  minus  1000)  apparent 
kilowatts,  showing  the  greater  proportionate  improvement 
secured  by  a  given  amount  of  leading  current  when  the 
power  factor  of  the  entire  system  is  low. 

Synchronous  motors  of  the  polyphase  type  are  separately 
excited.  No  series  winding  or  automatic  compounding  is 
required. 

When  generators  provided  with  compound  winding  are 
used  as  synchronous  motors,  it  will  be  found  that  increased 
separate  excitation  is  required,  since  the  series  fields  are 
necessarily  omitted  in  the  motor.  The  standard  exciters 
usually  furnished  with  generators  of  this  type  under  250 
kilowatt  capacity  must,  as  a  rule,  be  replaced  by  the  next 
larger  sizes.  The  following  table  gives  the  average  exciting 
current  required  by  standard  compound  wound  polyphase 
generators  and  synchronous  motors  of  moderate  output, 
power  factor  being  taken  as  unity.  The  exciter  capacities 
given  are  sufficient  to  take  care  of  inductive  conditions. 
The  exciters  should  have  ample  capacity,  as  they  are  a 
vital  part  of  the  system,  and  should  not  be  taxed  to  their 
full  capacity.  In  a  station  where  several  synchronous 
motors  or  generators  are  used,  it  is  customary  to  install  two 
independently  driven  exciting  dynamos,  each  one  of  which 
has  capacity  to  furnish  sufficient  excitation  for  all  machines. 


POLYPHASE   APPARATUS    AND   SYSTEMS. 


Exciting1  Current  and  Exciters  for  Standard  Polyphase  Genera- 
tors and  Synchronous  Motors. 


25-60  ( 

CYCLES. 

GENERATOR  OR  MOTOR. 
RATING  K.  W. 

SEPARATE 
EXCITING 
CURRENT. 

VOLTAGE. 

EXCITER 
CAPACITY. 

(  Generator  .  . 

6 

25 

1  1/2  K.W. 

(  Motor    .... 

10 

25 

2     K.W.  to  2y2  K.W. 

(  Generator  .  . 

8 

25 

2     K.W.  to  2i/2  K.W. 

1    I  Motor    .... 

!3 

25 

254  K.W. 

(  Generator  .  . 

IO 

25 

2y2  K.W. 

100-  <  , 
I  Motor    .... 

15 

25 

2%  K.W. 

(  Generator  .  . 

12 

125 

2y2  K.W. 

'H  Motor   .... 

20 

I25 

3y2  K.W.  to  4%  K.W. 

(  Generator  .  . 
25<H  Motor    .... 

19 
O"* 

I25 
125 

4%  K.W. 
7y2  K.W. 

Power  Factor.  — The  maximum  efficiency  of  the  motor 
and  circuit  exists  when  the  current  and  E.M.F.  supplied 
to  the  motor  are  in  phase  —  i.e.,  when  the  power  factor  is 
unity.  This  is  also  a  condition  of  minimum  current,  and 
the  drop  in  the  line  is  that  due  to  ohmic  resistance  only. 
When  the  current  is  in  advance  of,  or  lagging  behind,  the 
impressed  volts,  the  power  factor  is  less  than  unity.  It  is 
possible,  as  we  have  seen,  so  to  adjust  the  exciting. current 
of  a  synchronous  motor,  that  its  power  factor  may  be 
unity  at  any  load.  In  this  way  a  low  power  factor  of  the 
supplying  circuit,  due  to  induction  motors,  may  be  raised 
any  amount. 

On  account  of  armature  reaction,  a  motor  which  has  its 
excitation  adjusted  to  give  a  power  factor  of  unity  at  full 
load,  will  take  at  all  points  below  full  load,  a  leading 
current,  and  have  a  power  factor  less  than  100  per  cent. 


SYNCHRONOUS    MOTORS.  153 

For  the  average  case,  it  will  be  found  most  desirable  so  to 
excite  the  motor  fields  that  the  minimum  current  and 
highest  power  factor  are  reached  at  about  average  load. 
The  power  factor  will  be  leading  at  lighter,  and  lagging  at 
greater,  loads.  Except  in  the  case  of  synchronous  motors 
of  abnormally  bad  design,  the  power  factor,  with  properly 
excited  fields,  will  have  a  high  value  over  a  wide  range  of 
load.  Even  motors  with  considerable  armature  reaction 
will  have  only  a  slightly  drooping  curve  of  power  factor. 

Motors  which,  as  generators,  would  have  excellent  in- 
herent regulation  —  i.e.,  small  armature  reaction  and  self- 
induction  —  can  be  made  to  have,  with  one  adjustment  of 
the  field,  practically  100  per  cent  power  factor  at  all  but 
light  loads.  The  advantage  of  a  more  uniform  power 
factor  in  such  motors  is  offset  by  their  instability  during 
voltage  fluctuations.  Some  self-induction  is  desirable  in 
order  to  prevent  exchange  of  current  between  motor  and 
lines  when  the  impressed  volts  vary,  as  often  happens  in 
power  transmissions.  In  selecting  a  synchronous  motor, 
therefore,  preference  should  be  given  to  that  one  which,  as 
a  generator,  would  not  have  very  close  inherent  regulation. 
Machines  of  not  such  good  regulation,  have,  as  a  rule,  a 
higher  efficiency,  and  take  less  starting  current. 

To  predetermine  the  proper  field  strength  which  will 
give  the  maximum  condition  of  efficiency,  it  is  necessary 
to  know  the  conditions  of  the  system,  —  the  reactance  of 
the  generator  and  line,  the  average  load  and  its  power  fac- 
tor, and  the  characteristics  of  the  motor.  Each  case  is  a 
problem  by  itself,  and  must  be  judged  by  the  special  con- 
ditions affecting  it. 

A  synchronous  motor  will  take  no  more  than  its  rated 
amperes  without  overheating,  whatever  the  phase  relation 


154 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


of  current  and  E.M.F.  may  be.  If  the  inductive  load  at 
the  receiving  end  is  large  as  compared  with  the  capacity 
of  the  motor,  the  synchronous  motor  may  be  unable  to 
raise  the  power  factor  of  the  system  appreciably  while 
driving  its  own  load. 

It  will  be  found  that,  for  every  load  and  every  power 


L/A  E  EFFICIL  NCY  CURVES 


70  00          50          40  30  20 

POWER  FACTOR  (RECEIVING  END)  ' 


10 


Fig.  88. 

factor,  there  is  a  synchronous  motor  capacity  which  will 
make  the  efficiency  of  the  system  a  maximum.  Mr.  E.  J. 
Berg  has  calculated  the  influence  of  synchronous  motors 
upon  the  efficiency  of  alternating  systems.  Fig.  88  shows 
the  different  efficiencies  of  a  transmission  of  a  constant 
current  of  200  amperes  when  a  synchronous  motor  of  50, 
100,  or  150  kilowatts  is  running  as  a  compensator  at  the 


SYNCHRONOUS    MOTORS.  155 

receiving  end,   which  is  assumed  to  have  varying  power 
factors.     The   circuit   is   supposed   to   have   the  following 

constants : 

Current  =  200  amperes. 

Resistance  =  0.52  ohm. 

Reactance  =  1.45  ohms. 

Voltage  at  motor  =  1000. 

It  will  be  noticed  that,  as  the  power  factor  diminishes  at 
the  receiving  end,  the  line  efficiency  is  increased  by  using 
the  larger  synchronous  motors  —  i.e.,  will  transmit  a  greater 
amount  of  energy  for  the  same  loss.  The  line  efficiency  is 
greatest  when  using  the  150- kilowatt  motor  at  all  power 
factors  below  87.  The  line  efficiency  is  improved  by  en- 
tirely dispensing  with  the  motors  when  the  power  factor  is 
greater  than  95.  The  leading  current  of  the  motors  is 
then  in  excess  of  the  lagging  current  of  the  receiving  cir- 
cuit, thereby  increasing  the  total  current,  or  when  main- 
taining a  constant  current,  as  in  the  present  case,  decreasing 
the  energy  current  —  i.e.,  the  amount  of  power  that  can  be 
transmitted  over  the  lines  with  the  conditions  as  given. 

Instability.  —  When  a  number  of  synchronous  motors 
are  running  at  the  end  of  a  long  transmission  line,  it  is 
sometimes  found  that  a  condition  of  instability  exists,  espe- 
cially during  load  variations.  This  is  shown  by  an  inter- 
change of  synchronizing  current.  This  difficulty  may  in 
part  be  overcome  by  under-exciting  the  motor  fields,  which 
gives  a  lagging  armature  current. 

The  instability  or  "hunting"  of  synchronous  motors 
may  also  be  caused  by  the  periodic  variation  in  the  angu- 
lar velocity  of  the  prime  mover.  The  permissible  limits 
of  angular  variation  have  been  fixed  principally  because 
of  the  effect  on  the  operation  of  synchronous  apparatus  of 


156     POLYPHASE  APPARATUS  AND  SYSTEMS. 

changes  in  frequency  of  the  supply  current  during  each 
revolution  of  the  generator.  Any  disturbance  distorting 
the  motor-field  flux,  thereby  pulling  ahead  or  retarding  the 
revolving  element,  will  increase  the  tendency  to  hunt. 
This  tendency  is  more  noticeable  in  motors  of  high  volt- 
age when  designed  with  armatures  having  few  slots.  It 
may  be  reduced  by  various  forms  of  anti-hunting  devices, 
such  as  copper  rings  surrounding  the  poles,  by  heavy  metal 
bridges  between  the  poles,  or,  better  still,  by  the  amortis- 
seur  winding  previously  described. 


TRANSFORMERS,  157 


CHAPTER  VII. 
TRANSFORMERS. 

TRANSFORMERS  for  use  on  polyphase  circuits  may  be  either 
ordinary  single-phase  transformers  suitably  grouped,  or 
may  be  of  a  distinct  type  wound  polyphase.  Polyphase 
transformers  usually  have  as  many  magnetic  circuits  as 
there  are  phases,  although  the  two-phase  transformer  is 
sometimes  made  with  three  magnetic  circuits  connected  on 
the  three-wire,  two-phase  system.  In  the  polyphase  trans- 
former, since  the  flux  is  not  a  maximum  in  all  phases  at  the 
same  time,  the  iron  is  used  to  better  advantage  than  in 
separate  single-phase  transformers  and  less  is  required  for 
the  same  output. 

Until  quite  recently  the  polyphase  transformer  has  had 
only  limited  use  in  this  country,  American  engineers  for  the 
most  part  having  favored  an  appropriate  combination  of 
single-phase  transformers  for  all  the  commercial  polyphase 
systems.  This  preference  for  the  single-phase  transformer 
has  been  based  largely  on  the  simple  construction  and 
greater  flexibility  of  the  single-phase  type,  especially  because 
in  the  three-phase  system  damage  to  one  transformer  does 
not  interrupt  the  continuity  of  polyphase  transformation, 
so  that  with  three  transformers  normally  in  service  practi- 
cally two-thirds  of  the  load  can  be  carried  by  two  of  the 
transformers  in  the  event  of  damage  to  the  third.  A  three- 
phase  transformer  can,  with  proper  construction,  be  made 


158     POLYPHASE  APPARATUS  AND  SYSTEMS. 

nearly  as  convenient  in  this  respect  as  the  single- phase  com- 
bination by  cutting  out  the  damaged  phase  in  the  event  of 
injury.  The  advantage  of  three-phase  transformers  in  the 
matter  of  smaller  floor  space  and  lower  cost  are  now  caus- 
ing them  to  be  viewed  with  greater  favor  than  formerly,  and 
they  are  being  used  in  increasing  numbers.  The  saving 
in  cost  amounts  usually  to  more  than  10  per  cent,  and  the 
floor  space  may  be  less  than  three-quarters  of  that  required 
for  an  equal  output  in  the  single-phase  type. 

The  increasing  size  of  electrical  units  required  by  modern 
conditions  has  demanded  the  construction  of  very  large 
transformer  units  in  both  three-phase  and  single-phase 
types,  and  in  these  certain  features  of  construction  not 
found  in  the  smaller  units  are  embodied.  These  relate 
primarily  to  the  methods  for  getting  rid  of  the  heat  gener- 
ated in  the  core  and  windings.  In  small  transformers  the 
heat  can  be  readily  dissipated  by  natural  radiation  from  the 
exterior  of  the  transformer  casing.  The  radiating  sur- 
face of  a  transformer  increases  as  the  square  of  its  linear 
dimensions,  while  its  mass,  which  increases  directly  with  the 
output,  varies  as  the  cube  of  the  dimensions.  For  this 
reason  the  increase  of  radiating  surface  does  not  keep  pace 
with  the  increase  of  output,  and  a  capacity  is  soon  reached 
where  artificial  means  must  be  resorted  to  for  the  dissipation 
of  the  heat. 

The  ordinary  transformer  of  moderate  capacity  is  cooled 
by  being  immersed  in  oil.  The  heat  generated  in  the  coils 
and  iron,  taken  up  by  the  oil,  is  transmitted  to  the  iron  cas- 
ing and  is  thence  dissipated  by  radiation.  Transformers  of 
this  type  are  rarely  built  of  larger  size  than  100  to  150  kilo- 
watts. When  higher  capacities  than  these  are  reached 
either  the  radiating  surface  must  be  made  very  large,  requir- 


TRANSFORMERS.  159 

ing  a  large  tank  and  a  great  quantity  of  oil,  or  by  a  liberal 
use  of  copper  and  iron  the  transformer  losses  must  be  made 
very  low,  or  a  combination  of  these  expedients  must  be 
adopted,  resulting,  under  either  alternative,  in  a  rapid  in- 
crease in  cost.  Transformers  of  large  capacity  must 
accordingly  have  some  special  means  of  getting  rid  of  the 


Fig.  89. 

heat  generated  within  them.  A  number  of  methods  of 
cooling  are  employed,  but  all  transformers  may  be  classed 
as  belonging  either  to  the  self-cooled  or  artificially-cooled 
type.  It  will  be  more  satisfactory,  however,  to  discuss 
the  various  types  under  their  descriptive  names,  bearing  in 
mind  that,  apart  from  the  plurality  of  magnetic  circuits  and 
windings,  the  construction  of  polyphase  transformers  follows 


i6o 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


generally  the  same  details  as  are  presented  in  single-phase 
transformers. 

Self -cooled  Oil  Transformers.  —  The  ordinary  small  capac- 
ity transformer  is  of  the  self -cooled  type.  The  magnetic 
circuit  is  usually  a  plain  rectangle  of  interlaced  strips  of 
iron,  permitting  a  simple  form  of  winding.  Fig.  89  shows 
the  elements  of  such  a  transformer  of  the  single- phase  type. 
After  the  coils  are  assembled  on  the  two  vertical  legs  of  the 


no 


70 


eo 

.£ 

I50 


Time  in  hours 

Fig.  90. 

core  the  laminated  iron  piece  seen  at  the  bottom  of  the 
figure  is  connected  across  the  top  of  the  two  cores,  complet- 
ing the  magnetic  circuit.  The  primary  and  secondary  coils 
are  wound  concentrically  with  an  insulating  diaphragm  and 
oil  space  between  the  two.  The  laminated  iron  forming 
the  vertical  portion  of  the  core  is  built  up  in  cruciform 
section,  the  recesses  at  the  corners  providing  channels 
for  the  circulation  of  the  oil.  A  twofold  advantage  is 
gained  by  the  use  of  oil;  first,  the  temperature  is  reduced 


TRANSFORMERS.  l6l 

by  offering  a  ready  means  of  escape  for  the  heat;  second, 
punctures  in  the  insulation  are  immediately  repaired  by 
the  inflow  of  the  oil. 

The  reduction  in  temperature  by  the  use  of  oil  is  shown 
in  Fig.  90.  Curve  i  gives  the  rise  in  temperature  of  a 
transformer  not  submerged  in  oil,  as  determined  by  the  in- 
crease of  resistance  method.  Curve  2  shows  the  tern- 


Fig.  91. 

perature  rise  of  the  same  transformer  immersed  in  oil. 
Curve  3  shows  the  temperature  of  the  oil.  Curve  4  is  the 
temperature  of  the  windings  of  another  transformer  of 
poorer  design.  Curve  5  shows  the  temperature  of  this 
transformer  as  determined  by  thermometer.  This  last 
curve  does  not  give  the  true  average  heating,  for  the  ther- 
mometer cannot  reach  the  inaccessible  and  hottest  portions 
of  the  transformer. 


162 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


In  transformers  of  about  50  kilowatts  and  over,  the  con- 
taining case  is  usually  ribbed  or  fluted,  in  order  to  provide 
additional  radiating  surface.  The  coils  and  core  should 
also  present  a  large  surface  to  the  oil  so  as  to  permit  the 
heat  readily  to  be  conveyed  away  from  the  internal  parts. 
Fig.  91  illustrates  a  150  kilowatt  transformer  of  this 
type  built  by  the  Westinghouse  Company.  The  windings 


Fig.  92. 


are  divided  into  a  number  of  coils,  which  are  spread  apart 
at  the  ends,  thus  presenting  a  large  surface  to  the  oil. 
The  heat  generated  in  the  iron  and  in  the  coils  is  readily 
communicated  to  the  oil.  The  heated  fluid  rises,  flows 
to  the  top,  and  down  the  sides  of  the  case,  which  is  deeply 
ribbed  (see  Fig.  92,  showing  this  transformer  assembled 
in  its  case),  thus  presenting  a  large  surface  to  the  air. 


TRANSFORMERS. 


163 


The  coils  and  core  of  a  three-phase,  200  kilowatt  trans- 
former of  different  design  are  shown  in  Fig.  93.  The 
case  of  this  transformer  (not  shown)  is  also  of  the  fluted 
type. 

The  essential  features  of  large  capacity,  self-cooled  oil 
transformers,  are  high  efficiency,  corresponding  to  a  mini- 


Fig.  93. 

mum  energy  loss  to  be  dissipated  in  the  form  of  heat; 
an  internal  construction  which  presents  the  maximum 
surface  of  iron  and  windings  to  the  oil;  an  arrangement  of 
parts  which  permits  easily  to  be  established  the  convection 
currents  of  the  heated  oil ;  and  lastly,  a  ribbed  or  fluted  case, 
to  present  the  maximum  radiating  surface  to  the  external 
air. 


164     POLYPHASE  APPARATUS  AND  SYSTEMS. 

Water-cooled  Oil  Transformers.  —  When  some  method 
for  artificially  cooling  the  oil  is  provided,  the  generated  heat 
is  removed  without  the  aid  of  natural  radiation.  The  size 
of  tank  and  quantity  of  oil  may  therefore  be  chosen  with 
reference  only  to  the  dimensions  of  the  transformer  core 
and  windings  and  without  regard  to  the  amount  of  surface 
to  be  exposed  to  the  air.  Also,  since  the  amount  of  heat 
which  may  be  developed  in  the  transformer  is  not  limited 
to  the  value  which  can  be  dissipated  by  natural  radiation,  a 
greater  amount  of  heat  may  be  tolerated.  The  temperature 
of  the  parts  may  be  kept  within  safe  limits  since  the  oil  is 
kept  at  a  low  temperature  by  external  means.  From  the 
first  of  these  considerations  it  follows  that  smaller  tanks 
and  less  oil  may  be  used;  from  the  second  that,  where  advis- 
able, a  higher  copper  and  iron  loss,  that  is,  a  construction 
using  less  copper  and  iron  for  the  same  output  may  be 
used.  Hence,  in  sizes  which  commercially  may  be  con- 
structed in  either  the  self-cooled  or  the  artificially-cooled 
types,  that  is,  between  about  100  kilowatts  and  500  kilo- 
watts, the  artificially-cooled  type  will  have  the  advantage  of 
lower  cost.  This  advantage  is  proportionately  greater  in 
the  larger  sizes.  Above  500  kilowatts  the  self-cooled  type 
is  practically  non-existent,  since  not  only  its  cost  attains 
virtually  prohibitive  figures  but  also  the  problem  of  hand- 
ling the  losses  becomes  a  most  difficult  one.  The  general 
statement  may  be  made  that  practically  all  transformers 
over  250  kilowatts  now  constructed  are  of  the  artificially- 
cooled  type. 

Several  methods  of  cooling  have  been  employed.  In  one 
type  of  transformer  a  supply  of  cold  water  is  made  to  circu- 
late through  thin  flat  ducts  interposed  between  the  wind- 
ings. A  variation  of  this  arrangement  is  found  in  some 


TRANSFORMERS. 


I6S 


low  voltage  transformers  of  large  capacity,  built  a  few 
years  ago  for  electric  furnace  work.  The  secondary  wind- 
ing of  these  transformers  was  constructed  of  flat  copper 
tubes  through  which  the  circulation  of  water  was  maintained. 

Another  form  of  transformer  is  cooled  by  means  of  a 
water  jacket,  surrounding  the  case  containing  the  trans- 
former proper. 

In  a  third  form  the  method  of  cooling  consists  of  drawing 


Fig    94. 

off  the  oil,  cooling  it,  and  pumping  it  back.  A  motor, 
pump,  and  system  of  oil  tanks  for  circulating  and  cooling 
the  oil  are  used  to  control  the  temperature  of  the  trans- 
former (see  Fig.  94).  The  oil  is  forced  upward  through 
space's  left  around  and  between  the  coils,  overflows  at  the 
top,  and  passes  down  over  the  outside  of  the  iron  laminations. 
The  advantages  of  this  method  are  considered  to  lie  in  the 
fact  that  a  brisk  current  of  cold  oil  is  brought  directly  into 
contact  with  the  heated  transformer  parts  in  such  a  way  as 


1 66 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


to  absorb  the  heat  most  efficiently  and  quickly.  The  arrange- 
ment is  one  which  would  normally  find  application  only  in 
the  larger  plants  where  the  added  cost  of  the  cooling  devices 
would  be  warranted  by  the  saving  in  the  cost  of  the  trans- 


Fig  95. 

formers.  This  method,  which  of  recent  years  has  been  but 
little  used,  is  being  again  brought  forward,  and  a  number  of 
very  large  transformers  have  lately  been  constructed  on  this 
principle.  These  transformers  are  of  the  three-phase  type, 


TRANSFORMERS.  l6/ 

7500  kilowatts  each,  and  are  believed  to  be  the  largest  trans- 
formers ever  constructed. 

The  method  most  commonly  employed  cools  the  oil 
by  means  of  a  worm  or  system  of  pipes  inside  the  trans- 
former. A  transparent  view  of  such  a  transformer  is  shown 
in  Fig.  95.  A  supply  of  cold  water  is  circulated  through 
the  pipes,  seen  at  the  top.  Their  location  just  beneath 
the  surface  of  the  oil  places  them  in  the  most  favorable 
position  for  carrying  away  the  heat,  for  at  this  point  the 
difference  of  temperature  between  the  cooling  pipes  and 
the  heated  upper  layers  of  oil  is  greatest.  Strong  convection 
currents  are  thus  set  up,  streams  of  freshly  cooled  oil  con- 
stantly flowing  down  along  the  walls  of  the  tank  to  take  the 
place  of  the  heated  oil  which  rises  from  the  core  and  coils 
in  the  center. 

In  the  transformer  shown,  which  is  one  of  2000  kilowatts 
at  60,000  volts,  the  core  and  windings  are  suspended  from 
the  cover,  so  that  the  internal  parts  of  the  transformer 
may  readily  be  withdrawn  for  inspection.  The  tank  is  of 
heavy  steel  plate,  riveted  and  caulked,  and  a  valve  in  the 
base  permits  the  oil  to  be  withdrawn  if  necessary.  The 
ends  of  the  cooling  pipe  where  connection  is  made  to  the 
discharge  and  supply  pipes  are  taken  out  through  the  cover 
of  the  transformer. 

In  Fig.  96  are  shown  the  internal  parts,  less  cooling  pipe, 
of  a  transformer  of  the  same  general  design  but  of  the  three- 
phase  type.  This  transformer  is  rated  at  2200  kilowatts, 
60,000  volts. 

The  standard  construction  of  the  Westinghouse   Com- 
pany is  illustrated  by  Fig.  97,  which  shows  one  of  the  3000 
kilowatt    water-cooled   transformers   used   by  the    Ontario 
Power  Company  in  transmitting  power  from  Niagara  Falls. 


168 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


This  transformer  employs  a  triple  system  of  cooling  pipes 
of  which  the  supply  and  discharge  connections  are  seen  at 
the  right  and  left  respectively. 


Fig.  96. 

Some  outside  source  of  power  is  required  to  operate  the 
cooling  devices,  which  slightly  reduces  the  total  efficiency  of 
the  transformation.  The  water  coil,  may  be  conveniently 
supplied  by  water  mains,  or,  in  the  case  of  a  water-power 
transmission,  by  the  water  under  head. 


TRANSFORMERS.  169 

Oil  Type  Transformers  for  High  Voltage  Testing.  —  A 

special  type  of  oil  transformer  designed  to  give  the  very 
high  voltages  required  in  certain  testing  work  is  illustrated 
in  Fig.  98.  This  transformer  is  capable  of  producing  a 


Fig.  97. 

maximum  sustained  potential  of  250,000  volts.  Distin- 
guishing features  of  the  design  are  the  subdivision  of  the 
high  potential  winding  so  as  to  reduce  the  voltage  per  coil, 
and  the  special  form  of  terminals,  which  are  built  up  of 
varnished  fabric  tubes  and  filled  with  oil.  This  transformer, 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


as  well  as  most  others  intended  for  similar  work,  is  of  the 
core  type,  with  the  high  tension  winding  wound  outside  the 
low  tension  coils.  The  core  type  adapts  itself  more  readily 
to  the  problem  of  high  insulation,  which  in  these  trans- 


Fig.  98. 

formers  is  one  of  extreme  difficulty,  and  which  is  satisfac- 
torily met  only  by  a  liberal  spacing  of  the  parts  and  the  use 
of  the  best  materials  obtainable. 

Quality  of  Oil  Required  for  Transformer  Use.  —  Oil  for 
use  in  transformers,  particularly  at  high  voltage,  requires 
special  care  in  preparation,  selection,  and  handling. 


TRANSFORMERS.  I/I 

The  two-fold  purpose  of  the  oil  as  a  heat  conveying,  and 
as  an  insulating  medium,  requires  that  it  shall  be  non- 
viscous,  so  that  rapid  convection  currents-  shall  be  readily 
maintained,  and  that  it  shall  have  high  insulating  properties. 
Other  important  characteristics  are  a  high  flash-point  and 
an  absence  of  acid  or  alkaline  reaction.  Experience  has 
shown  that  these  qualities  are  possessed  only  by  pure  min- 
eral oils  of  certain  grades,  specially  refined  for  this  use. 

While  for  the  most  efficient  cooling  action  the  oil  should 
be  as  thin  as  possible,  the  ordinary  kinds  of  very  light  oil 
are  not  suitable,  for  the  reason  that  such  are  liable  to  have 
low  flashing  and  burning  points,  and  may  be  ignited  by  a 
heavily  overloaded  or  defective  transformer,  or  by  light- 
ning; or  when  heated  may  give  off  inflammable  vapors. 
The  two  requirements  of  high  fluidity  and  low  flashing 
point  are  in  a  sense  opposed,  yet  by  care  in  manufacture  it 
has  been  possible  to  meet  both  of  these  in  a  satisfactory 
manner;  and  instances  are  very  rare  where  oil  transformers 
made  by  the  best  manufacturers  have  taken  fire  from  any 
cause. 

With  the  continued  increase  in  the  potentials  for  which 
transformers  were  wound,  the  use  of  oil,  first  adopted  only 
as  a  convenient  means  of  cooling,  was  found  to  be  a  neces- 
sity from  the  insulation  standpoint.  For  potentials  much 
over  35,000  volts  there  have  as  yet  been  found  no  insulating 
materials  suitable  for  use  in  transformer  construction  that 
will  withstand  the  high  dielectric  stress  except  as  aided  by 
immersion  in  oil.  It  is  the  use  of  oil,  therefore, :  that  has 
made  possible  the  increasingly  high  voltages  which  modern 
conditions  demand.  For  satisfactory  results  the  insulating 
qualities  of  the  oil  must  be  of  the  most  perfect  description. 
These  are  secured  only  by  removing  the  last  traces  of  water, 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


which  is  frequently  present  as  an  impurity.  Oil  as  fur- 
nished by  responsible  makers  of  transformers  for  use  with 
their  apparatus  has  been  tested  by  them  for  insulation  prop- 
erties and,  if  necessary,  brought  up  to  standard  quality  by 
careful  drying.  Good  oil  does  not  absorb  moisture  by  ex- 
posure to  the  air,  but  water  may  be  present  in  the  contain- 
ing vessels,  or  may  be  deposited  from  the  atmosphere  during 
rapid  changes  of  temperature,  or  the  transformer  tanks  and 
parts  may  be  damp  before  the  transformer  is  filled.  All 


Fig.  99. 

these  considerations  point  to  the  necessity  of  thoroughly  dry- 
ing the  transformer  before  the  tank  is  filled  and  of  exercising 
care  in  the  handling  of  the  oil.  When  it  is  considered  that 
even  one-tenth  of  one  per  cent  of  moisture  renders  the  oil 
unfit  for  use,  it  will  be  seen  that  the  condition  of  the  oil 
must  be  fully  ascertained  before  it  is  used.  This  can  be 
done  with  certainty  only  by  subjecting  it  to  high  potential 
break-down  tests  made  in  accordance  with  the  standards  of 
the  transformer  manufacturer.  If  the  oil  proves  deficient  it 
must  be  dried  and  again  tested  before  the  transformer  is 


TRANSFORMERS. 


173 


filled.  As  a  final  precaution,  a  sample  of  oil  taken  from  the 
filled  transformer  is  tested  before  the  transformer  is  con- 
nected into  circuit.  These  tests,  which  are  simply  and 
easily  made,  are  very  necessary  in  the  case  of  transformers 
for  extra  high  tension  circuits  —  say  20,000  volts  or  over  - 


Fig    100, 

and  may  with  advantage  be  made  where  the  apparatus  is 
intended  for  much  lower  voltages. 

Air  Blast  Transformers.  —  In  this  transformer  the  cooling 
is  effected  by  means  of  a  forced  current  of  air  circulating 
through  the  windings  and  core. 

The  primary  and  secondary  coils  are  separately  wound  on 


174 


POLYPHASE  .APPARATUS    AND    SYSTEMS. 


formers  and  insulated,  and  then  assembled  in  groups  (Fig, 
99),  the  coils  being  intermingled.  The  groups  are  assem- 
bled in  the  form  of  a  case,  being  separated  from  one  another 
by  vertical  air  spaces.  The  magnetic  circuit  is  then  built 
up  around  the  windings  (Fig.  100),  horizontal  ducts  being 
provided  at  frequent  intervals  in  the  laminations. 

It  is   evident   that   this   construction   permits   the   most 


Fig.  101. 

complete  ventilation,  as  the  very  heart  of  the  transformer  is 
reached  by  the  blast  of  air.  The  flow  of  air  is  controlled 
by  means  of  two  dampers,  one  of  which  is  located  at  the 
top  of  the  transformer,  regulating  the  air  between  the  wind- 
ings; the  other  is  on  the  side  of  the  frame,  and  controls  the 
flow  of  air  through  the  core.  Fig.  101  shows  the  arrange- 
ment of  iron  and  copper  parts  and  ventilating  ducts,  and 
Fig.  1 02  a  completed  transformer  in  its  frame. 


TRANSFORMERS.  175 

The  apparatus  for  furnishing  the  air  blast  consists  of  a 
blower,  and  is  usually  operated  by  an  induction  motor,  the 


Fig.  102. 


air  being  delivered  to  the  transformer  by  means  of  a  flue. 
The  volume  of  air  required  for  cooling  purposes  varies 
with  the  number,  size,  and  efficiency  of  the  transformers. 


176 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


About  150   cubic   feet  of  air  per  minute  is  required  per 
kilowatt  loss  in  the  transformer. 

The  following  table  gives  the  volume  and  pressure  of 
air  required  for  transformers  of  various  sizes  of  the  average 
efficiency : 


Total  K.W.  of 
Transformer. 

*O  <ub^ 
II 

Cubic  Ft.  Air 
Required  Per 
Transformer 
Per  Min. 

U  j^ 

•~  ^ 

11 
s  b 

O  2* 

c  <u 

Frequency  of 
Circuit. 

^sj 
J5 

Speed  of 
Blower  and 
Motor. 

H.P.  to  Drive 
Blower  Full 
Volume  and 
Pressure 

900 

100 

45° 

4,05° 

6,000 

i 

25 

5°' 

75° 

2-5 

40 

5°' 

800 

60 

40' 

900 

1,  800 

2OO 

900 

8,100 

8,000 

f 

25 

55' 

75° 

4 

40 

55' 

800 

60 

5o' 

900 

2,700 

300 

1,125 

10,125 

10,000 

f 

25 

55' 

75° 

5 

. 

40 

55' 

800 

60 

55' 

720 

4,5°° 

500 

1,625 

14,625 

14,000 

f 

25 

75' 

500 

6-5 

40 

7°' 

600 

60 

7°' 

720 

6,75° 

75° 

1*875 

16,875 

20,000 

f 

25 

90' 

500 

12 

40 

80' 

600 

60 

So' 

600 

7>5°° 

1,250 

2,800 

16,800 

2O,OOO 

f 

25 

90' 

500 

12 

40 

80' 

600 

60 

80' 

600 

From  the  table  it  will  be  seen  that  the  power  consumed 
in  cooling  the  transformers  is  scarcely  one-tenth  of  one  per 
cent  of  the  output  of  the  transformers.  If  the  transformers 
have  an  efficiency  of  97.5  per  cent  at  full  load,  the  total 
efficiency  of  transformation  is  reduced  to  97.4  per  cent  by 
the  use  of  the  air  blast  —  a  perfectly  negligible  quantity. 

In  case  of  damage  to  the  cooling  arrangement,  the  trans- 
formers can  operate  for  a  short  period  without  the  air  blast, 
but  as  the  transformers  are  not  designed  to  dissipate  their 


TRANSFORMERS. 


177 


losses  by  natural  radiation,  their  continued  operation 
requires  an  uninterrupted  supply  of  air  from  the  blower,  and 
conservative  practice  demands  that  the  blower  set  should 
therefore  be  provided  in  duplicate. 

Air-blast  transformers  are  successfully  used  on  voltages 
up  to  35,000  volts.  At  potentials  much  above  this  the 
difficulties  of  insulation  are  considerably  enhanced  and  can- 
not satisfactorily  be  met  at  present  except  in  the  oil  type 
of  transformer. 

In  capacities  below  100  kilowatts,  the  design  of  the  air- 
blast  transformer  is  not  favorable  to  minimum  cost,  and  in 
most  cases  the  self-cooled  oil  type  will  be  found  cheaper. 
For  this  reason  the  air-blast  type  is  mostly  built  in  sizes 
above  100  kilowatts.  It  is  well  adapted  to  the  largest 
capacities,  and  though  units  having  an  output  of  3000  kilo- 
watts, single-phase,  are  probably  as  large  as  have  yet  been 
constructed,  larger  sizes  are  entirely  feasible. 

The  approximate  weight  of  transformers  of  this  type  is 
given  in  the  following  table,  which  covers  low  frequency 
and  high  frequency  designs: 


AIR-BLAST   TRANSFORMERS. 


Weight  in  Pounds. 

Capacity 

K.W. 

25  Cycles. 

60  Cycles. 

100 

3,800 

3,000 

15° 

5,20O 

4,000 

200 

6,400 

5,000 

300 

8,300 

6,500 

500 

1  1,  600 

9,100 

75° 

15,100 

12,000 

1,000 

18,  100 

14,400 

1,500 

23,600 

19,000 

POLYPHASE  APPARATUS  AND  SYSTEMS. 


Operation  of  Air-Blast  Transformers.  —  When  trans- 
formers of  this  type  are  run  in  groups  or  "  banked  "  together, 
care  should  be  taken  that  the  air  enters  each  transformer  at 
the  same  pressure,  otherwise  the  transformers  will  take  un- 
equal amounts  of  air  and  heat  unequally.  This  can  be 
accomplished  by  having  the  flue  or  blast-chamber  of  such 
cross  section  that  the  velocity  of  air  will  not  exceed  200  feet 


Fig.  103. 


per  minute.  The  most  desirable  installation  of  the  trans- 
former is  over  a  closed  chamber  of  liberal  size,  which 
besides  keeping  the  air  velocity  within  suitable  limits,  will 
give  the  added  advantage  of  permitting  ready  inspection  of 
the  windings  and  connections.  Unequal  air  pressure  in 
different  transformers  may  be  compensated  for  by  means  of 
the  two  dampers.  The  temperature  of  the  outgoing  air 
affords  a  ready  means  of  determining  the  proper  amount  of 


TRANSFORMERS. 

air  to  be  admitted  to  each  transformer.  The  supply  is 
normally  sufficient  if  the  outgoing  air  is  not  more  than  20 
degrees  C.  hotter  than  the  surrounding  atmosphere.  Fig. 
103  shows  the  installation  and  connections  of  single-phase, 
air-blast  transformers  in  a  long  distance  power  transmission. 

Structure  of  Magnetic  Circuit.  —  It  will  have  been  noticed 
from  the  illustrations  that  so  far  as  concerns  the  arrange- 
ment of  the  coils  with  respect  to  the  magnetic  circuit,  all  the 
transformers  described  conform  to  one  or  the  other  of  two 
general  types.  These  are  known  generally  as  the  "core" 
type,  in  which  the  coils  are  assembled  around  a  central  core 
(see  Fig.  89),  and  the  "shell"  type,  in  which  the  magnetic 
circuit  surrounds  the  coils,  as  in  Fig.  100.  Practically  all 
transformers  now  built  may  be  classed  as  belonging  to  one 
or  the  other  of  these  two  well  denned  types. 

In  general,  all  small  transformers,  say  up  to  one  or  two 
hundred  kilowatts,  are  built  in  the  core  type.  Among  the 
advantages  of  this  form  is  the  simplicity  of  construction, 
which  permits  easy  assembly  or  dismantling;  also,  due  to 
the  cylindrical  shape  of  the  winding,  the  "mean  length  of 
turn"  is  small,  which  reduces  the  resistance  and  improves 
the  regulation. 

In  large  sizes  the  core  type  results  in  an  excessive  length 
of  magnetic  circuit,  and  it  also  becomes  troublesome 
properly  to  support  the  vertically  disposed  coils  so  as  to 
avoid  crushing  the  bottom  turns.  Most  transformers  of 
large  output  are  therefore  built  in  the  shell  type,  which  in 
large  capacities  has  the  advantage  of  lower  cost  and  a  prefer- 
able mechanical  arrangement.  The  distinguishing  charac- 
teristics of  this  type  are  the  double  magnetic  circuit  of  short 
length,  a  small  magnetizing  current,  and  a  disposition  of  the 
coils  which  reduces  the  magnetic  leakage  to  a  minimum. 


i8o 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Efficiency  and  Losses.  —  The  characteristic  efficiency 
curve  of  a  well-designed  transformer  shows  a  high  efficiency 
at  all  but  very  light  loads.  In  Fig.  104  the  efficiency  of  a 
25o-kilowatt,  6o-cycle  transformer  is  above  90  per  cent  even 
at  outputs  as  low  as  about  -£$  load.  Good  efficiency,  at 
light  loads,  is  a  valuable  feature,  especially  in  motor  and 
lighting  transformers,  where  the  average  load  rarely  makes 


100 

90 

I 

£    80 


50 


Mo 


Full 


Per  Cent,  of  Load 
Fig.  104. 


a  demand  of  more  than  one-half  of  the  transformer  capacity. 
The  efficiencies  taken  from  the  curve,  are  as  follows: 

TVload      .    ,  ~.    .    . 87       percent 

J  load      94-6  per  cent 

i  load      .    .   .V.   .    •:  •   « 97       percent 

f  load 97.  7  per  cent 

Full  load 98       per  cent 

ij  load 98.  i  per  cent 

The  losses  in  a  transformer  consist  only  of  copper  and 
iron  losses.     The  former  vary  with  the  load,  while  the  iron 


TRANSFORMERS. 


181 


or  core  losses  remain  about  the  same  for  all  loads.  It  is 
necessary,  therefore,  in  order  to  obtain  good  efficiency  at 
light  loads,  to  reduce  the  core  losses  to  a  minimum.  Judg- 
ing from  the  shape  of  the  curve  in  Fig.  104,  the  core  loss  must 
be  small.  This  is  shown  to  be  the  case  in  Fig.  105,  which 
gives  the  watts  lost  in  the  iron  of  the  same  transformer,  and 
also  the  corresponding  exciting  current. 

At  full  voltage  the  exciting  current  is  2.3  amperes.  The 
exciting  current  is  the  current  which  the  transformer  takes 
at  no  load,  with  normal  voltage  and  frequency  impressed. 
It  is  the  vector  sum  of  two  currents,  viz.,  the  current  required 
to  produce  the  flux  in  the  iron,  called  the  magnetizing  cur- 
rent, and  the  current  consumed  by  the  core  loss.  The  lat- 
ter is  an  energy  current  and  is  in  phase  with  the  E.M.F. 
and  with  the  energy  component  of  the  load.  The  magnet- 
izing current  is  purely  wattless  and  lags  90  degrees  behind 
the  E.M.F.  Ordinarily 
the  magnetizing  current 
will  be  about  three- 
fourths  of  the  exciting 
current,  and  the  core  loss 
current  about  two-thirds 
of  the  exciting  current, 
bearing  in  mind  that  it 
is  the  vector  sum  of  these 
two  which  gives  the  ex- 
citing current.  From  this 
it  follows  that  with  the 

magnitudes  assumed  the  lag  angle  of  the  exciting  current  is 
that  one  having  0.66  for  its  cosine,  or  an  angle  of  about  50 
degrees.  The  total  current  input  at  any  load  is  the  vector 
sum  of  the  exciting  current  and  the  current  (lagging,  lead- 


2400 

1 
3380 

2300 

^ 

2000 

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1600 

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1200 

X 

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400 

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4 

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3 

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400  £00  1200  ItiOO  2000  2400  2800  3200  360G 

Fig.  105. 


1 82     POLYPHASE  APPARATUS  AND  SYSTEMS. 

ing,  or  in  phase)  required  by  the  load.  In  a  well  designed 
transformer  the  exciting  current  is  small,  so  that  the  vector 
sum  referred  to  is  but  little  greater  than  the  load  current. 
Thus  the  exciting  current  adds  but  little  to  the  copper  loss 
in  a  good  transformer.  A  high  exciting  current,  in  addition 
to  increasing  the  current  input  at  all  loads,  means  that  the 
transformer  will  have  poor  regulation,  especially  at  low 
power  factor. 

Again  referring  to  the  curves  (Fig.  105),  the  core  loss  of 
the  transformer  in  question  is  seen  to  be  3380  volts,  or  1.3 
per  cent  of  the  full  load  input.  Since  with  a  full-load  effi- 
ciency of  98  per  cent  as  shown  by  the  preceding  curve,  the 
total  losses  are  2  per  cent  of  the  input,  the  loss  in  the  copper 
conductors  is  the  difference,  or  0.7  per  cent.  By  reducing 
the  amount  of  copper  in  both  primary  and  secondary  coils, 
say  one-half,  we  double  the  loss  in  the  copper,  making 
it  now  1.4  per  cent,  and  reducing  the  efficiency  of  the 
transformer  to  97.3  per  cent.  But  the  total  cost  of 
the  transformer  is  thereby  decreased  by  from  10  to  20 
per  cent. 

It  is  not  always  wise  to  select  the  more  efficient  trans- 
former, especially  in  water-power  transmissions,  where  a 
large  item  in  the  cost  of  delivered  power  is  the  interest  on 
the  plant,  nor  in  plants  where  there  is  not  a  demand  for 
every  horse  power  developed.  As  an  illustration,  take  the 
case  of  a  power  transmission  of  1000  H.P.  using  the  cheaper 
transformer,  which  has  an  efficiency  of  97.3  per  cent.  The 
power  delivered  is  about  1.5  per  cent,  or  15  H.P.,  less  than 
with  the  more  efficient  step-up  and  -down  transformers.  If 
a  market  were  found  for  every  horse  power  transmitted  at, 
say  $30  per  horse  power  per  year,  the  loss  in  revenue  to  the 
power  company  would  be  $450  a  year.  As  a  partial  offset, 


TRANSFORMERS.  183 

there  would  oe  the  interest  on  the  difference  in  the  first  cost 
of  the  transformers.  Few  water-power  transmissions,  how- 
ever, are  run  at  their  full  capacity.  When  such  is  the  case, 
the  power  company  is  usually  warranted  in  buying  the  ex- 
pensive transformer.  In  the  transmission  of  steam-gener- 
ated power,  fuel  is  generally  the  most  important  single 
factor  in  the  make-up  of  the  total  cost  of  power,  and,  as  a 
rule,  the  most  efficient  transforming  devices  should  be 
used. 

Regulation.  —  Regulation  in  a  transformer  is  the  per- 
centage drop  of  secondary  voltage  from  no  load  to  full  load, 
the  primary  voltage  remaining  constant.  Stated  in  another 
way,  the  regulation  is  the  percentage  difference  between  the 
full-load  and  the  no-load  ratio.  Good  regulation  is  more 
desirable  in  a  transformer  than  in  a  generator,  as  there 
are  no  means  of  compounding  a  transformer  for  voltage 
drop.  Modern  constant  potential  transformers  of  the  best 
makes,  however,  regulate  with  great  closeness,  the  regu- 
lation seldom  exceeding  i  per  cent  except  in  very  small 
units. 

On  non-inductive  load  the  regulation  is  substantially 
equal  to  the  percentage  IR  drop  in  the  windings,  or,  which 
is  the  same  thing,  to  the  percentage  PR  loss  of  the  trans- 

77? 

former,  since  the  value  of  —  ,  which  is  the  regulation,  must 

hi 

PR 
be  the  same  as  the  value  of     -  ,  which  gives  the  percentage 

copper  loss. 

The  actual  regulation  at  non-inductive  load  is  somewhat 
higher  than  would  follow  from  the  foregoing,  for  the  reason 
that  the  assumptions  made  do  not  take  into  account  the 
reactance  of  the  transformer  windings.  In  a  transformer 


1 84 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


having  a  resistance  drop  of  say  0.5  to  i.o  per  cent  the  react- 
ive drop  will  be  from  2  per  cent  up  to  4  or  5  per  cent  or  even 
higher,  depending  on  the  reactance  of  the  transformer.  A 
reasonable  approximation  to  the  non-inductive  regulation 
is  obtained  by  combining  at  right  angles  the  resistance  and 
reactance  drop  due  to  the  main  current.  The  vector  sum 
of  these  is  the  total,  or  impedance,  drop,  which  combined 
with  the  impressed  E.M.F.  in  the  proper  phase  relation  will 
show  the  resultant  E.M.F. ,  from  which  the  regulation  is 
deduced. 

Referring  to  Fig.  106  for  the  approximate  conditions  apply- 
ing at  non-inductive  load,  we  have  OE,  taken  equal  to  100 
per  cent,  representing  the  secondary  E.M.F.  of  the  trans- 
former at  full  load.  In  phase  with  OE  is  EB,  equal  to  the 

IR  drop  of  primary  plus 
secondary  expressed  in 
percentage  of  normal 
volts.  At  right  angles 
to  EB  is  E0B,  equal  to 
the  reactive  drop  at  full 
load,  expressed  similarly  as  a  percentage.  Therefore  EEQ 
is  the  impedance  drop  through  the  transformer  windings. 
The  resultant  of  OE  and  EEQ  is  OE0,  which  is  thus  the  value 
of  E.M.F.  which  must  be  impressed  to  give  a  full-load 
voltage  equal  to  OE.  If  there  were  no  drop,  resistance  or 
reactive,  the  delivered  voltage  would  be  the  same  as  the 
impressed,  or  equal  to  OE0,  which  can  thus  be  considered 
as  the  no  load  delivered  E.M.F.  Hence  the  per  cent 
regulation,  which  is  referred  to  the  no-load  voltage,  is 


Fig,  106. 


OE.  —  OE 


TRANSFORMERS.  185 

With  an  inductive  load  at  80  per  cent  power  factor  lag- 
ging, the  conditions  are  represented  by  Fig.  107.  At  80  per 
cent  power  factor  the  cur- 
rent /  lags  about  37  de- 
grees behind  the  E.M.F, 
The  IR  drop,  equal  as 
before  to  EB,  is  in  phase  Fi 

with  the  current  /.     The 

reactive  drop  E^B  is  90  degrees  behind  the  current  pro- 
ducing it.  Completing  the  triangles,  the  regulation  is 
deduced  according  to  the  same  formula  as  before.  It  is 
obvious  that  the  difference  of  magnitude  between  OE0  and 
OE  is  considerably  greater  than  in  the  previous  figure, 
showing  graphically  the  greater  voltage  drop  (i.e.,  poorer 
regulation)  with  lagging  current.  Conversely,  a  leading 
current  with  sufficient  angle  of  advance  will  result  in  a 
negative  regulation,  that  is,  a  higher  voltage  at  full  load 
than  at  no  load. 

Parallel  Operation.  —  For  successful  parallel  operation 
all  the  transformers  should  have  the  same  ratio  of  trans- 
formation and  the  same  regulation.  Any  transformer  in 
which  the  ratio  is  low,  which  means  that  the  transformer 
will  give  too  high  a  voltage  on  the  load  side,  will  tend  to 
set  up  a  local  flow  of  current  between  itself  and  the  trans- 
formers which  give  a  lower  voltage  on  the  load  side.  Full 
load  current,  or  even  more,  may  thus  be  made  to  circulate 
between  the  transformers,  depending  upon  the  difference 
in  ratio.  This  will  be  the  condition  applying  at  no  load. 
When  it  is  attempted  to  take  load  from  two  transformers  of 
unequal  ratio  which  are  connected  in  multiple,  all  the 
current  delivered  to  the  external  circuit  will  be  furnished 
by  the  transformer  which  tends  to  give  the  higher  voltage, 


1 86 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


until  the  voltage  drop  in  this  transformer,  with  the 
increased  current  flow,  is  such  as  to  bring  its  terminal 
voltage  down  to  equality  with  that  of  the  other  trans- 
former. The  transformers  giving  higher  voltage  will 
therefore  under  all  conditions  of  load  be  carrying  more 


Fig.  108, 

than    their    share    of    current,     with     consequent    over- 
heating. 

Any  transformers  having  a  poorer  regulation  than  the 
others  will  deliver  lower  voltage  as  the  load  comes  on,  and 
thus  take  less  than  their  share  of  the  load,  thereby  putting 
extra  load  on  the  other  transformers  and  causing  them  to 
overheat. 


TRANSFORMERS.  187 

There  is  no  remedy  for  inequality  of  ratio  except  to  make 
the  ratio  correct  by  some  means,  as  by  bringing  out  taps 
from  the  windings.  Inequalities  of  regulation  can,  how- 
ever, be  adjusted  by  the  use  of  external  resistance,  or, 
preferably,  reactance,  in  series  with  the  transformers  whose 
regulation  is  too  close,  the  external  drop  thus  obtained  act- 
ing to  make  the  effective  regulation  enough  poorer  to  result 
in  a  satisfactory  division  of  the  load. 

Change  of  Ratio  of  Transformation.  —  Variations  in  vol- 
tage are  sometimes  necessary,  as,  for  instance,  in  the  use  of 
rotary  converters,  where  it  may  be  desired  to  vary  the  direct- 
current  E.M.F.  over  a  wide  range.  This  variation  must, 
broadly  speaking,  be  obtained  by  a  corresponding  variation 
in  the  E.M.F.  of  the  alternating  current.  One  method  of 
varying  this  E.M.F.  is  to  have  the  transformers  so  con- 
structed as  to  permit  a  change  in  the  ratio  of  primary  to 
secondary  turns.  This  is  accomplished  by  bringing  out 
taps  or  loops  from  the  windings  and  connecting  them  to  a 
dial  switch  so  arranged  that  the  number  of  active  turns, 
and  thus  the  E.M.F.,  is  varied  by  moving  the  switch  to 
connect  with  successive  taps.  Fig.  108  illustrates  a  three- 
phase,  air-blast  transformer  arranged  in  this  manner. 
Each  phase  has  its  own  dial  switch,  all  three  being  actuated 
by  a  single  hand  wheel,  so  that  the  voltage  increase  or  de- 
crease is  effected  equally  in  all  three  legs  of  the  circuit. 


188     POLYPHASE  APPARATUS  AND  SYSTEMS. 


CHAPTER  VIII. 
ROTARY  CONVERTERS. 

General.  —  A  rotary  converter  is  essentially  a  continuous- 
current  generator  which,  in  addition  to  its  commutator, 
is  equipped  with  two  or  more  collector  rings  connected  to 
symmetrical  points  in  the  armature  winding.  If  such  a 
machine  be  driven  by  an  external  source  of  power  it  will 
evidently  deliver  either  alternating  or  continuous  current,  or 
both.  If  supplied  with  electric  power  it  will  operate  either 
as  a  synchronous  alternating- current  motor,  as  a  continuous- 
current  motor,  or  as  a  converter  of  alternating  current  into 
continuous  current,  or  vice  versa.  Such  machines  are  ordi- 
narily employed  to  convert  alternating  into  direct  current, 
in  which  case  the  alternating  current  enters  the  armature 
winding  through  the  collector  rings  and  is  delivered  as 
continuous  current  after  being  rectified  by  the  commutator. 

When  used  in  the  opposite  sense,  that  is,  to  convert  con- 
tinuous current  into  alternating  current,  the  continuous 
current  enters  the  armature  winding  through  the  commuta- 
tor, the  alternating  current  being  taken  off  at  the  collector 
rings.  When  used  in  this  way  the  designation  "inverted 
converter"  is  commonly  applied,  or  it  is  said  that  the  con- 
verter is  running  inverted. 

As  usually  designed,  rotary  converters  differ  but  little  in 
mechanical  construction  and  in  general  appearance  from 
direct-current  generators.  This  will  be  seen  from  Figs.  109 


ROTARY   CONVERTERS.  189 

and  no,  which  represent  two  views  of  a  Westinghouse  1500 
kilowatt  converter.  In  the  design  of  apparatus  of  this 
type,  however,  certain  values  for  armature  reaction  and  for 
other  constants  have  been  found  by  experience  to  be  desir- 
able, and  these  values  in  general  differ  considerably  from 
the  values  of  corresponding  constants  in  continuous-current 
generator  design.  While,  therefore,  a  continuous-current 
generator  may  be  made  into  a  rotary  converter  by  the  addi- 
tion of  armature  taps  and  collector  rings,  in  the  manner 
described,  it  is  usually  not  desirable  to  do  so  for  the  reason 
that  without  some  change  in  the  proportioning  of  parts  and 
windings  a  rotary  converter  so  made  would  probably  not 
give  the  best  results.  Furthermore,  the  number  of  poles  and 
the  speeds  for  which  continuous- current  generators  are  ordi- 
narily constructed  correspond  to  lower  frequencies  than  are 
commercially  used.  For  example,  the  frequency  of  typical 
slow  speed  continuous-current  generators  is  usually  be- 
tween 10  and  15  cycles  per  second,  so  that  it  would  only  be 
occasionally  possible  to  make  from  standard  generators 
rotary  converters  adapted  to  commercial  frequencies. 

If  the  taps  from  the  armature  to  the  collector  rings  are 
taken  out  at  points  180  electrical  degrees  apart,  the  machine 
becomes  a  single-phase  rotary  converter.  Connections  at 
points  90  degrees  apart  make  a  two-phase  or  quarter-phase 
converter,  this  connection  being  essentially  a  double  single- 
phase  arrangement  with  90  degrees  displacement  between 
the  phases.  If  the  connections  are  made  at  points  120  or 
60  degrees  apart  respectively,  the  machine  is  a  three-phase 
or  a  six- phase  converter. 

Connections.  —  The  various  connections  of  the  alternat- 
ing end  of  rotary  converters  are  diagrammatically  shown  in 
Figs,  in  to  118,  which  also  show  various  practical  methods 


i  go 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


of  connecting  converters  and  transformers  together.  The 
circle  at  the  bottom  of  each  diagram  represents  the  arma- 
ture winding  of  a  bipolar  converter,  the  collector  rings  being 
omitted  for  simplicity. 

Fig.  in  shows  the  connections  for  a  two-phase  converter 
fed  by  two  single-phase  transformers.     The  same  sketch, 


Fig.  109. 

omitting  one  transformer  and  its  connections,  would  repre- 
sent the  connections  of  a  single-phase  converter. 

In  Figs.  112  and  114  are  shown  the  connections  of  a  three- 
phase  converter  with  the  transformer  secondaries  connected 
"delta"  and  "Y"  respectively. 

Fig.  115  shows  the  diametrical  connection  of  transformers 
for  a  six-phase  converter.  As  in  the  case  of  the  two-phase 


ROTARY   CONVERTERS. 


IQI 


converter,  the  secondary  terminals  of  a  given  transformer 
are  connected  to  points  in  the  armature  180  electrical  de- 
grees apart.  As  the  two-phase  converter  is  fed  by  two 
single-phase  circuits  differing  in  phase  by  90  degrees,  so  the 
six-phase  converter  is  fed  by  three  single-phase  circuits 
differing  in  phase  from  each  other  by  60  electrical  degrees. 


Fig.  110. 

This  is  the  simplest  method  of  connecting  a  six-phase  con- 
verter with  its  transformer  and  has  the  advantage  that  trans- 
formers with  single  secondary  coils  can  be  used. 

Other  connections  are  also  sometimes  used,  as  the  six-phase 
delta  and  six-phase  "  Y,"  shown  in  Figs.  116  and  118,  both 
of  which  require  two-coil  secondaries  in  the  transformers. 
It  will  be  noted  that  the  delta  or  "Y"  six-phase  connection 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


is,  in  effect,  two  three-phase  connections  applied  to  the 
same  armature,  the  connections  of  one  set  being  reversed. 
This  is  true  also  of  the  six-phase  "T"  connection,  which 
is  a  double  reversed  three-phase  "T"  connection,  as  shown 
by  Figs.  113  and  117.  The  "T"  connection  is  rarely  used, 


'WVW\ 


Two-Phase. 
Fig.  111. 


Uvwwwvv  Uvwvwvw  VWWWAW 
13 


Three -Phase  A 
Fig.  112. 


3 


VWVWWVW  WM/VWVW   \WWWWW 


'Three -Phase  T 
Fig.  113. 


Three  -  Phase  Y 
Fig.  114. 


as  it  provides  no  features  of  especial  merit  that  are  not 
equally  obtainable  by  simpler  arrangements. 

In  multipolar  rotary  converters  each  collector  ring  is  con- 
nected to  as  many  points  of  the  armature  as  there  are  pairs 
of  poles,  that  is,  the  connections  must  be  duplicated  for  each 
360  electrical  degrees  of  the  armature. 

Ratio  of  Alternating   to  Direct-Current   Voltage.  —  The 


ROTARY    CONVERTERS. 


193 


alternating- current  voltage  of  the  rotary  converter  is  always 
less  than  the  continuous-current  voltage,  the  value  of  which 
latter  is  equal  to  the  crest  of  the  E.M.F.  wave,  while  the 
alternating-current  voltage  corresponds  to  the  mean  effec- 


IwywwvwJ  Iwwwww    UwvwvwJ 


Six. -Phase  Diametrical- 
Fig.  115. 


WWvWWW    VWVWVWW   VVWVWWw 
|3 


Six-Phase 
Fig.  116. 


wwwwwv  vwwvwvw  wwwwvw 


[pAMA/Ww^W^AAAMJL 


Six-Phase  T 
Fig.  117. 


Six-Phase  Y 
Fig.  118. 


tive  value  of  the  wave.  The  E.M.F.  induced  in  the  wind- 
ing of  any  direct-current  generator  is  alternating  in  character 
and  is  rectified  by  the  commutator  when  the  impulses  are 
at  their  maximum.  The  effective  value  (root  mean  square) 

of  this  alternating  E.M.F.  is  — p  =  0.707  of  the  E.M.F.  at  the 

V2 

commutator  brushes,  where  the  alternating  E.M.F.,  as  in 
the  case  of  a  single-phase  converter,  is  measured  across  an 


194     POLYPHASE  APPARATUS  AND  SYSTEMS. 

electrical  diameter,  or  180  electrical  degrees.  This  is  the 
relation  between  the  alternating  and  continuous  voltage  of 
a  single-phase,  of  a  two-phase,  or  of  a  six-phase  diametrical 

connected  rotary  converter. 

If  the  alternating  E.M.F. 
is  measured  across  some 
other  portion  of  the  arma- 
ture, this  value  will  be  lower. 
Where  the  alternatin  E.M.F. 


c 

is  applied  across  120  elec- 
trical degrees,  the  ratio  of 
voltage  will  be  that  deducible 
from  the  diagram  shown  in 
Fig.  119. 

Taking  the  value  of  the  continuous  current  E.M.F.  as 
equal  to  unity,  let  the  diameter  AC  be  equal  to  the  meas- 
ured value  of  the  alternating  diametrical  E.M.F.,  viz.,  —p  • 

V2 

ABC  is  a   right  triangle  being  inscribed  in  a   semicircle. 
Further,  BC,  which  is  a  chord  subtending  60  degrees,  is 

equal  to  the  radius,  or  to  — ;=•  •  , 

2  V2 

Therefore  AB  =  J(-^\  -  (-^Y  =\/|=  0.613. 

*     \V2    /  \2xv  2/  *    ° 

This  value,  0.613,  is  therefore  the  theoretical  ratio  for  a 
three-phase  converter. 

These  ratios  are  the  theoretical  values  applying  at  no 
load.  At  full  load  they  are  modified  by  the  magnitude  of 
the  IR  drop  .in  the  armature,  which  requires  a  higher  alter- 
nating E.M.F.  to  be  impressed  in  order  to  obtain  a  given 
E.M.F.  at  the  direct-current  brushes.  The  effect  of  this  IR 


ROTARY    CONVERTERS.  195 

drop  is  most  noticeable  in  converters  of  low  voltage.  The 
theoretical  ratios  both  at  no  load  and  at  full  load  are  more- 
over not  always  obtained  in  practice,  the  departures  from 
the  theoretical  values  being  due  to  a  variety  of  causes  act- 
ing alone  or  in  conjunction.  Among  these  causes  are  the 
percentage  of  armature  circumference  covered  by  the  pole 
faces;  the  position  of  the  direct-current  brushes  on  the 
commutator;  the  shape  of  the  potential  wave  furnished  by 
the  circuit  from  which  the  converter  is  operated;  and  the 
amount  of  excitation. 

As  the  direct-current  voltage,  neglecting  the  ohmic  drop 
in  the  converter,  is  equal  to  the  maximum  instantaneous 
alternating  E.M.F.  (measured  diametrically)  that  is,  to 
the  crest  of  the  alternating  wave,  it  will  be  seen  that  a 
flat  top  wave  gives  a  lower  direct-current  voltage  with  the 
same  impressed  alternating  voltage,  that  is,  a  higher  ratio, 
and  a  peaked  wave  under  the  same  conditions  gives  a 
higher  direct-current  voltage,  that  is,  a  lower  ratio.  Fur- 
ther, the  shape  of  the  alternating  E.M.F.  wave  impressed 
by  the  generator  upon  the  converter  is  modified  by  the 
counter  E.M.F.  wave  of  the  converter.  A  short  pole  arc 
on  the  converter,  producing  a  flat  top  counter  E.M.F.  wave, 
thus  tends  to  lower  the  direct-current  voltage  at  the  same 
impressed  alternating  voltage,  while  a  long  pole  arc  tends  to 
raise  it.  That  this  effect  is  of  marked  importance  is  seen 
from  the  example  of  a  recently  built  six-phase  machine  of 
loco  kilowatts  capacity.  In  this  converter  a  pole  arc  of 
75  per  cent  was  used  and  the  ratio  was  found  to  be  0.725. 
Upon  inserting  longer  pole  faces,  corresponding  to  a  pole 
arc  of  80  per  cent,  the  ratio  then  became  0.685.  ^n  most 
commercial  machines  the  percentage  pole  arc  will  range 
between  70  per  cent  and  80  per  cent,  and  between  these 


196 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


values  a  given  change  in  the  percentage  pole  arc  will  produce 
nearly  the  same  percentage  change  in  the  conversion  ratio. 

Under  average  conditions  of  operation  the  standard  types 
of  rotary  converters  will  have  at  full  load  approximately 
the  ratios  shown  in  the  following  table,  in  which  for  com- 
parison are  also  inserted  the  theoretical  no-load  ratios 
based  on  sine  wave  E.M.F. 


Type. 

Voltage. 

Full-Load 
Ratios. 

Theoretical 
No-Load 
Ratio. 

Two-phase  a 
rical)  .  . 

tid  six-phase  (diamet- 

("550  volts 
\  2tjo  volts 

0.725 
072 

0.707 
o  707 

Three-phase 
delta) 

and  six-phase  (Y  or 

(125  volts 
(  550  volts 

<   2<O   VOltS 

°-735 
o.  62 
o  62  < 

o.  707 

0.613 

(125  volts 

0.63 

o.  613 

These  values  of  the  full-load  ratio  apply  to  converters  of 
about  25  to  30  cycles.  Converters  for  higher  frequencies, 
as  50  or  60  cycles,  having  a  larger  number  of  poles,  are 
built  with  a  greater  distance  between  pole-tips  to  minimize 
magnetic  leakage,  in  other  words,  with  a  smaller  percentage 
pole  arc,  and  will  in  general  have  slightly  higher  ratios. 

If  the  direct -current  brushes  are  shifted  in  either  direction 
from  the  neutral  point,  the  direct-current  voltage  is  lowered. 
A  change  of  several  per  cent  in  the  ratio  may  be  caused  in 
this  way,  but  the  amount  of  variation  possible  is  limited  by 
the  amount  of  brush  shift  that  is  permissible  within,  the 
limits  of  satisfactory  commutation. 

A  change  in.  the  excitation  will  also  affect  th,e  ratio  slightly. 
When  the  fields  are  strongly  excited  the  direct- current  vol- 
tage will  be  somewhat  increased  by  reason  of  the  action  of 
the  leading  current  on  the  reactance  of  the  armature  wind- 
ings; conversely,  a  weak  excitation  which  produces  lagging 


ROTARY    CONVERTERS.  197 

current  will  cause  the  direct- current  voltage  to  drop.  This 
effect  is,  however,  trivial,  providing  that  the  alternating 
voltage  at  the  collector  rings  is  maintained  constant,  and 
should  not  be  confounded  with  the  effect  of  leading  and 
lagging  current  traversing  reactance  external  to  the  con- 
verter, whereby  the  collector-ring  voltage  and,  with  it,  the 
continuous-current  voltage,  is  increased  and  diminished. 
This  latter  effect  will  be  referred  to  again  in  connection  with 
the  subject  of  regulation  of  voltage  by  field  excitation. 

In  the  normal  operation  of  the  converter,  i.e.,  when 
furnishing  direct  current,  the  ohmic  drop  reduces  the  direct- 
current  voltage.  When  run  as  an  inverted  converter,  i.e., 
when  delivering  alternating  current  — direct  current  being  fed 
to  the  brushes  —  the  drop  is  on  the  alternating  side;  conse- 
quently the  ratio  of  a  converter  is  lower  when  run  inverted. 

For  preliminary  calculations  where  the  data  of  operation 
are  not  known,  the  ratios  given  in  the  preceding  table 
may  be  used  for  most  standard  converters,  though  the 
present  tendency  is  to  construct  machines  with  such  length 
of  pole  arc  as  will  give  at  full  load  about  the  ratios  corre- 
sponding to  the  no-load  values  given. 

Where  rotary  converters  are  installed  in  several  sub- 
stations at  different  distances  from,  the  generating  station 
and  fed  from  the  same  transmission  lines,  the  nearer  sub- 
stations will  obviously  receive  the  alternating  current  at 
a  higher  potential  than  will  those  more  remotely  located. 
In  such  cases  it  is  customary  to  equip  the  step-down  trans- 
formers with  taps  in  the  high-tension  winding  so  that,  by 
selecting  the  proper  tap  the  same  secondary  potential  may  be 
delivered  in  each  of  the  substations,  thus  insuring  a  uniform 
direct-current  voltage  over  the  system.  A  range  of  10  per 
cent  is  the  customary  allowance  in  the  high-tension  trans- 


198     POLYPHASE  APPARATUS  AND  SYSTEMS. 

former  taps,  commonly  arranged  in  four  steps  of  2^  per  cent 
each.  These  taps,  besides  serving  the  purpose  described, 
are  sometimes  very  useful  in  compensating  for  inequalities 
of  the  conversion  ratio  of  different  converters  operated  in 
parallel  in  the  same  station. 

Ratio  of  Alternating  to  Continuous-Current  Amperes.  - 
Considering  the  internal  losses  as  nil,  a  comparison  of  the 
voltage  ratios  in  the  several  types  of  converter  will  show 
that  the  following  values  apply  to  the  relation  between  the 
continuous  current  and  the  alternating  current  flowing  in 
each  phase: 


Type  of  Converter. 

Relative  Value 
A.C.to  B.C. 

Two-phase 

O    7O7 

Three-phase        .                    . 

O    0-13 

Six-phase  (delta,  Y,  diametrical  or  T) 

0.472 

These  approximate  values  hold  for  sine-wave  current 
only  and  for  unity  power  factor.  If  the  alternating  current 
is  not  in  phase  with  the  E.M.F.  the  above  values  must  be 
divided  by  the  power  factor  in  order  to  arrive  at  the  correct 
ratio. 

The  value  of  the  ratios  given  in  the  table  affords  a  rough 
means  for  determining  whether  the  converter  is  operating  at 
approximately  unity  power  factor,  for  under  this  condition, 
as  will  be  seen,  the  three-phase  current  is  about  equal  to  the 
continuous  current,  the  two-phase  is  about  three-quarters  of 
the  continuous,  and  the  six-phase  about  one-half  the 
continuous. 

Relative  Economy  of  Material  in  Different  Types.  —  When 
in  a  rotary  converter  alternating  current  is  being  transformed 
into  continuous  current,  or  vice  versa,  the  action  of  the 


ROTARY   CONVERTERS. 

machine  may  be  regarded  as  consisting  of  a  motor  action  and 
a  generator  action  taking  place  simultaneously  in  the  same 
windings.  Hence,  the  two  actions  partly  neutralize  each 
other,  since  the  alternating  current  flowing  in  a  given  con- 
ductor is  opposed  by  the  continuous  current  flowing  in  the 
same  conductor,  and  the  current  actually  flowing  at  a  given 
moment  in  a  conductor  is  approximately  equal  to  the  dif- 
ference between  the  value  of  the  continuous  current  and  the 
instantaneous  value  of  the  continually  changing  alternating 
current.  This  net  value  in  a  polyphase  converter  is  always 
less  than  the  value  which  would  flow  in  the  conductors  if  the 
converter  were  operated  as  a  generator  driven  mechanically 
from  an  external  source  of  power.  From  this  it  follows  that 
for  the  same  PR  loss  the  armature  conductors  of  a  poly- 
phase converter  can  be  made  of  smaller  cross  section  than 
in  a  generator  of  equal  output.  The  greater  the  number 
of  phases  for  which  a  converter  is  wound,  the  greater  will 
be  this  saving,  because  w'ith  many  phases  the  path  through 
the  armature  between  alternating  and  continuous  current 
terminals  is  shorter  and  more  direct.  The  approximate 
relative  amount  of  this  saving  is  indicated  in  the  following 
table,  which  shows  the  capacity  in  kilowatts  which  could 
be  delivered  by  various  types  of  converters,  assuming  that 
they  all  contain  the  same  weight  of  armature  copper  as  wrould 
be  required  in  a  generator  of  100  kilowatts  capacity,  which 
is  taken  as  the  basis: 


Type  of  Machine. 

Capacity. 

Continuous  current  generator     
Three-phase  converter      

IOO 

m 

Two-phase  converter    

161 

Six-phase  converter  

104 

2OO     POLYPHASE  APPARATUS  AND  SYSTEMS. 

Since  the  foregoing  table  is  based  on  considerations  of 
armature  copper  alone,  and  does  not  take  into  account  other 
electrical  or  mechanical  parts  of  the  machine  which  are  not 
affected,  it  follows  that  the  several  weights  of  a  given  con- 
verter when  designed  alternatively  for  two-phase,  three- 
phase,  or  six-phase  connection,  will  not  show  as  great  a 
difference  as  do  the  several  capacity  ratios  given  in  the 
table.  Nevertheless,  the  progressive  reductions  in  the 
amount  of  armature  copper  as  the  number  of  phases  is 
increased  operates  of  itself  to  reduce  the  aggregate  weight 
of  the  machine,  both  as  a  result  of  the  diminished  weight  of 
the  conductors  and  as  a  result  of  the  somewhat  smaller 
armature  diameters  which  can  be  used  to  advantage  when 
the  number  of  phases  is  increased.  Another  advantage 
secured  by  increasing  the  number  of  phases,  is  that  the 
tendency  to  pulsation  is  reduced  owing  to  the  more 
uniform  turning  moment,  which  follows  from  the  fact 
that  the  number  of  impulses  per  revolution  is  greater. 
An  increased  number  of  phases  tends  also  to  improved 
commutation. 

The  capacity  ratios  in  the  table  referred  to  are  based  on 
non-inductive  load.  Where  the  current  is  lagging  or  lead- 
ing, the  armature  heating,  for  the  same  continuous  current 
output,  is  increased,  and  the  capacity  ratios  thereby  pro- 
portionately reduced.  The  wattless  component  circulates 
through  all  conductors  in  series  between  phases,  and  thus 
over  a  path  of  higher  resistance  than  does  the  energy  com- 
ponent flowing  from  collector  rings  to  commutator.  A 
given  intensity  of  wattless  current  will  therefore  cause  a 
greater  loss  of  power  in  resistance  than  will  the  same 
intensity  of  energy  current.  From  this  is  explained  why, 
with  a  given  increase  in  the  alternating  current  input  due 


ROTARY   CONVERTERS. 


201 


to  phase  displacement,  the  increase  in  heating,  and  reduc- 
tion in  capacity,  is  greater  than  would  at  first  seem  to 
be  reasonable,  having  reference  only  to  the  relative  squares 
of  the  current  values  corresponding  to  the  non-inductive 
and  to  the  inductive  condition  respectively.  In  this  con- 
nection the  following  example  is  cited:  By  the  preceding 
table  the  capacity  ratio  for  a  three-phase  converter  at  non- 
inductive  load  is  1.31.  Assume  now  a  phase  displacement 
giving  a  wattless  current  equal  to  30  per  cent  of  the  input. 
The  total  current  being  100  per  cent,  the  energy  current, 
which  is  in  quadrature  with  the  wattless  current,  will  be 
95.4  per  cent.  We  should  expect  an  increase  of  heating  in 
the  ratio  of  ioo7  to  95.4.,'  or,  which  is  the  same  thing,  that  the 
capacity  for  the  same  heating  would  be  reduced  in  the  pro- 
portion of  95.4  to  100,  that  is,  by  4.6  per  cent.  If  this  were 
true  the  capacity  ratio  of  the  converter  would  become 

1.31  X  0.954  =  1.25. 

Instead  of  this  it  is  found  that  the  capacity  ratio  is  reduced 
over  8  per  cent,  or  to  1.20.  Steinmetz  (Elements  of  Electri- 
cal Engineering)  calculates  the  following  values  of  the 
capacity  ratio  with  30  per  cent  wattless  current.  The 
unity  power  factor  capacity  ratios  from  the  preceding  table 
are  repeated  for  comparison. 


Type  of  Machine. 

Capacity. 

30%  Watt- 
less 
Current  . 

Unity 
Power 
Factor. 

Continuous  current    

100 
120 

145 
170 

100 

131 
161 
194 

Three-phase  converter  
Two-phase  converter    
Six-phase  converter  

202     POLYPHASE  APPARATUS  AND  SYSTEMS. 

Since  30  per  cent  wattless  current  corresponds  to  a  power 
factor  of  95.4  per  cent,  it  is  seen  that  even  a  comparatively 
small  departure  from  non-inductive  conditions  causes  a 
marked  increase  in  the  resultant  heating.  At  90  per  cent 
P.F.,  corresponding  to  about  44  per  cent  wattless  current, 
the  -  condition  is  further  aggravated,  and  the  three-phase 
converter  capacity  ratio  thereby  reduced  to  1.16.  From 
this  it  is  evident  that  the  heating  of  a  converter  is  notably 
sensitive  to  changes  in  power  factor,  and  that  when  a  phase 
displacement  exists  the  reduction  in  effective  capacity  is 
more  marked  than  in  other  types  of  synchronous  machines. 
Hence,  where  converters  are  to  be  used,  like  synchronous 
motors,  for  phase  control,  it  is  important  that  due  provi- 
sions should  be  made  in  the  design. 

Regulation  of  Voltage.  —  Mention  has  been  made  of  the 
fact  that  so  long  as  the  impressed  alternating  E.M.F.  re- 
mains constant,  little  change  can  be  brought  about  in  the 
continuous  current  E.M.F. ,  either  by  change  of  excitation 
or  by  brush  shift.  Stated  in  another  way,  this  means  that 
the  voltage  conversion  ratio  of  a  given  machine  is  practi- 
cally a  fixed  quantity,  and  that  to  increase  or  decrease  the 
direct  current  E.M.F.  by  any  considerable  amount  a  corre- 
sponding increase  or  decrease  must  be  brought  about  in  the 
alternating  E.M.F. 

This  may  be  effected  by  the  use  of  a  potential  regulator 
inserted  in  the  alternating-current  leads.  This  method  is 
mostly  used  where  a  wide  voltage  variation  has  to  be  accom- 
plished while  the  converter  is  carrying  load,  and  where  a 
fine  and  smooth  adjustment  is  demanded,  as  in  lighting 
service. 

Another  method  calls  for  the  provision  of  taps  or  loops 
in  the  windings  of  the  transformers  which  feed  the  con- 


ROTARY   CONVERTERS.  2O3 

verter.  These  taps  give  a  step-by-step  change  in  the  trans- 
formation ratio  and  thus  a  corresponding  change  in  the  de- 
livered secondary  potential.  To  effect  quickly  the  desired 
adjustment  these  taps  are  connected  to  a  dial  switch  located 
on  or  near  the  transformer.  This  method,  like  that  referred 
to  in  the  preceding  paragraph,  can  be  employed  to  give  any 
desired  range  of  adjustment,  but  is  not  so  well  adapted  for 
use  when  the  converter  is  carrying  load  on  account  of  the 
sparking  which  takes  place  at  the  contacts  when  the  posi- 
tion of  the  switch  is  changed.  Unless  the  switch  is  designed 
to  pass  very  quickly  from  point  to  point,  there  is  also  the 
possibility  that  the  converter  may  fall  out  of  step  in  the 
interval  during  which  the  circuit  is  momentarily  opened. 
By  this  method  of  regulation  the  potential  is  changed  in  a 
series  of  abrupt  and  definite  steps  and  not  smoothly  as  with 
the  method  first  described;  and  while  the  number  of  taps 
may  be  made  sufficiently  great  to  give  only  a  small  varia- 
tion between  steps,  this  method  has  only  a  limited  applica- 
tion, for  the  reason  that  when  the  number  of  taps  is  made 
very  large  the  added  cost  and  complication  in  the  trans- 
former becomes  excessive.  * 

The  third  method  makes  use  of  the  property  which  the 
converter,  like  the  synchronous  motor,  possesses,  namely, 
that  of  producing  a  phase  displacement  by  change  of  field 
strength.  Thus  the  current  may  be  made  to  lag  or  to  lead 
according  as  the  field  is  under-excited  or  over-excited. 

Now,  the  E.M.F.  of  self-induction  lags  90  degrees  behind 
the  current  producing  it.  Hence,  with  lagging  current  and 
a  phase  displacement  of  90  degrees,  the  phase  of  the  E.M.F. 
of  self-induction  will  be  180  degrees  behind  that  of  the  im- 
pressed E.M.F.,  that  is,  directly  opposed  to  it,  and  the 
resultant  E.M.F.  will  be  the  difference  between  the  two. 


2O4     POLYPHASE  APPARATUS  AND  SYSTEMS. 

Similarly,  when  the  current  is  leading  by  90  degrees  the 
E.M.F.  of  self-induction  will  be  in  phase  with  the  impressed 
E.M.F.  and  additive  directly  thereto.  For  phase  displace- 
ments less  than  90  degrees  the  effect  will  be  similar,  though 
of  diminished  magnitude.  With  sufficient  self-induction 
and  with  the  amount  of  phase  displacement  feasible  to  ob- 
tain in  practice  it  is  possible  to  effect  by  this  means  a  poten- 
tial variation  of  from  six  to  eight  per  cent,  and  sometimes 
more,  under  favorable  conditions.  This  method  of  voltage 
control  will  be  considered  again,  and  with  a  numerical 
example,  in  a  later  paragraph. 

Rotary  converters  are  commonly  arranged  either  for 
shunt  or  for  compound- wound  fields.  In  the  former  case 
the  field  coils  are  connected  across  the  direct-current 
brushes,  while  in  the  latter  case  a  series  winding  is  pro- 
vided in  addition  to  the  shunt  excitation.  The  field  con- 
nections in  either  case  are  precisely  similar  to  those  of 
shunt  or  compound  wound  continuous-current  generators. 

Shunt-Wound  Converters.  —  In  the  shunt- wound  con- 
verter, for  any  given  setting  of  the  field  rheostat,  the 
field  strength  remains  practically  constant  for  all  condi- 
tions of  load.  Since  the  magnitude  of  any  phase  displace- 
ment, as  well  as  its  direction,  that  is,  lagging  or  leading, 
is  dependent  on  the  field  strength,  it  follows  that  in  this 
type  of  machine  any  given  phase  displacement  will  remain 
unchanged  irrespective  of  the  load  —  in  other  words,  that 
the  converter  will  operate  at  practically  constant  power 
factor.  This  power  factor  may  be  unity,  or  the  current 
may  be  made  always  lagging  or  always  leading,  dependent 
on  the  value  of  field  strength  given  to  the  converter.  This 
property  is  graphically  shown  in  Fig.  1 20,  which  represents 
several  phase  characteristics  of  a  100  kilowatt,  three-phase, 


ROTARY    CONVERTERS. 


205 


550  volt  machine.  Each  curve  shows  the  variation  in  the 
alternating-current  input  for  varying  field  strengths  at  dif- 
ferent loads,  the  continuous  current  output  for  the  condi- 
tions represented  by  any  particular  curve  being  maintained 
constant  at  the  value  appearing  directly  above  each  curve. 


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AMPERES  FIELD 
Fig.  120. 

The  field  strength  for  minimum  current  input,  or  100  per 
cent  power  factor,  is  0.92  ampere  at  no  load,  and  about 
0.95  ampere  when  the  continuous  current  output  has  been 
increased  to  203  amperes,  or  to  about  10  per  cent  overload 
(which  happens  to  be  the  load  at  which  the  highest  curve 
was  taken).  This  small  increase  in  field  current  necessary 


206 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


to  maintain  unity  power  factor  proves  that  the  armature 
reaction  is  very  slight. 

This  property  is  shown  in  another  way  by  the  field1  char- 
acteristic curve  of  the  same  100  kilowatt  converter,  repre- 
sented in  Fig.  121.  Through  a  range  of  output  between  no 
load  and  about  20  per  cent  overload  this  curve  shows  the 
increase  in  field  ampere  turns  necessary  to  maintain  unity 
power  factor,  the  potential  at  the  direct-current  brushes 


ouuu 

2500 
2000 
1600 
1000 
600 

1 

550 

Volts  L 

.  c  c\ 

nstant 

Potential 

0         80        40         60         80       100        120       140       160       180       200       220 
Amperes  D.  G> 

Fig.  121. 

being  maintained  constant  at  550  volts.  The  ampere  turns 
at  no  load  are  2700  and  at  an  output  of  203  amperes  are 
2800  —  an  increase  of  about  3.7  per  cent.  The  ratio  of 
increase  in  the  ampere  turns  is  of  course  the  same  as  the 
relative  increase  in  the  amperes  field,  as  shown  in  the  phase 
characteristic  of  Fig.  120,  both  curves  being  plotted  from 
the  same  tests. 

It  is  therefore  seen  that  the  characteristics  of  the  shunt- 
wound  converter  in  the  matter  of  keeping  the  power  factor 
practically  constant  over  wide  ranges  of  load  insure  that 


ROTARY    CONVERTERS.  2O/ 

the  delivered  direct-current  potential  shall  be  practically 
constant,  provided  that  constant  potential  be  delivered  to 
the  high  tension  side  of  the  step- down  transformers.  It 
follows  equally  that  variations  in  the  high-tension  alternat- 
ing pressure  will  reappear  in  the  form  of  proportionate 
variations  in  the  direct-current  voltage,  the  ratio  of  transfor- 
mation in  the  step-down  transformers  and  the  conversion 
ratio  of  the  converter  itself  both  being  practically  constant. 
Since  variations  in  the  high-tension  voltage  received  by 
the  transformers  are  caused  chiefly  by  changes  in  the  line 
drop  incident  to  fluctuations  in  the  load,  the  shunt-wound 
converter  is  not  therefore  the  advantageous  type  where  a 
constant  direct-current  pressure  is  desired  over  considerable 
ranges  of  fluctuating  load.  The  exception  to  this  is  the 
case  where  the  changes  of  load  are  gradual  enough  to  per- 
mit the  voltage  to  be  readjusted  by  hand,  by  any  of  the 
several  methods  referred  to  in  the  preceding  section.  To 
conditions  of  this  character  the  shunt-wound  type  is  well 
adapted,  and  it  is  therefore  frequently  used  on  large  sys- 
tems where  the  changes  of  load  take  place  slowly,  or  where 
it  is  important  to  maintain  constant  conditions  of  power 
factor. 

Compound-Wound  Converters. — In  the  compound-wound 
type  the  strength  of  the  field  increases  with  increase  of 
load,  due  to  the  added  ampere  turns  produced  by  the 
series  winding.  Hence,  if  at  no  load  the  shunt  field  is  ad- 
justed to  produce  a  lagging  current,  the  progressive  building 
up  of  the  field  as  load  comes  on  will  diminish  the  phase 
displacement  and  finally  bring  the  current  into  phase  with 
the  E.M.F.,  or  even  cause  it  to  lead  the  E.M.F.  if  the  series 
winding  is  strong  enough.  In  this  way,  by  altering  the 
angle  of  phase  displacement,  the  E.M.F.  of  self-induction 


208     POLYPHASE  APPARATUS  AND  SYSTEMS. 

combining  with  the  impressed  E.M.F.  will  tend  progres- 
sively to  raise  the  voltage  at  the  collector  rings  with  increase 
of  load,  thereby  effecting  a  corresponding  increase  in  the 
direct-current  potential. 

Compound-wound  converters  are  therefore  used  to  ad- 
vantage where,  with  fluctuating  loads,  it  is  desired  auto- 
matically to  control  the  voltage  delivered  from  the  direct- 
current  terminals.  The  conditions  of  railway  work  are 
usually  of  this  kind,  and  it  is  here  that  the  compound- 
wound  converter  is  most,  and  almost  exclusively,  employed. 

In  order  to  show  the  relation  of  the  various  factors  which 
give  these  results,  we  will  take  the  case  of  a  quarter-phase 
converter  designed  for  an  output  on  the  direct-current  side 
of  300  kilowatts  at  600  volts.  In  the  numerical  example 
which  follows  it  will  be  assumed  for  the  sake  of  simplicity 
that  constant  voltage  is  maintained  on  the  secondary  ter- 
minals of  the  transformers  from  which  the  converter  is  fed, 
and  that  the  various  data  are  as  follows: 

ROTARY  CONVERTER,  QUARTER-PHASE. 

Capacity     ............    ...  300     kilowatts 

Efficiency,  full  load  ...........  o.  93 

Conversion  ratio,  A.C.  -r-  D.C  .......  o.  71 

Volts,  D.C  ........    .    ......  600 

Volts,  A.C.  (600  X  0.71)     ........  426 

Volts  at  transformer  secondary  held  constant 

at     ................    ,  426 


A.C.   amperes   full   load,  aXo-93  -    37* 

0.07  X  300000 
A.C.  amp.  no  load,  -         x      5  24- 

Voltage  consumed  at  378  amp.  by  self-induc- 
tion of  circuit  between  secondary  terminals 
of  transformer  and  converter  collector  rings, 
assumed  to  be  equal  to  15  per  cent  of  second- 
ary E.M.F.  of  transformer,  or  0.15  X  426  =  64. 


ROTARY   CONVERTERS.  209 

Let  us  suppose  the  converter  to  be  running  at  no  load, 
that  is,  with  zero  output  from  the  D.C.  side,  and  with  the 
field  strength  adjusted  for  unity  power  factor.  Let  the 
field  now  be  weakened  until  the  current  input  rises  to,  say, 
30  per  cent  of  the  full-load  non-inductive  amperes,  or  to 
0.30  X  378  =  114  amperes.  Since  the  energy  current 
required  at  no  load  is  but  24.6  amperes  and  since  by 
producing  a  heavy  lagging  current  the  input,  still  at  no 
load,  has  been  raised  to  114  amperes,  it  follows  that  the 
power  factor  has  been  reduced  from  unity  to  0.216  (equal- 
ling 24.6  -f-  114)  and  that  the  angle  by  which  the  current 
is  lagging  behind  the  E.M.F.  is  that  angle  whose  cosine 
is  0.216,  or  about  77  degrees. 

We  are  now  ready  to  construct  the  diagram  of  Fig  122, 
which  shows  the  various  E.M.F.  's  of  one  phase. 


0 

Fig.  122. 

On  the  base  line  OE  draw  El  so  that  angle  OEI  equals 
77  degrees,  OE  representing  the  E.M.F.  impressed  on  the 
collector  rings,  and  El  representing  the  current  lagging  77 
degrees  behind  it.  Since  with  full-load  current  the  E.M.F. 
of  self-induction  is  64  volts,  and  since  at  no  load  the  current 
input  has  been  made  equal  to  30  per  cent  of  the  full-load  cur- 
rent, the  E.M.F.  of  self-induction  at  no  load  will  be  0.30  X 
64  =  19.2  volts.  EEQ  is  accordingly  drawn  90  degrees  be- 
hind El  and  equal  to  19.2. 


2IO     POLYPHASE  APPARATUS  AND  SYSTEMS. 

In  the  triangle  OEE0,  OE0  will  therefore  represent  the 
value  of  the  E.M.F.  which  the  transformer  secondary  must 
deliver  in  order  that  an  E.M.F.  equal  to  OE  shall  be 
delivered  to  the  collector  rings;  or,  which  is  the  same 
thing,  if  OEQ  is  drawn  equal  to  the  transformer  secondary 
E.M.F.,  or  426  volts,  the  length  of  OE  will  represent  the 
E.M.F.  that  is  delivered  to  the  collector  rings.  Solving 
the  triangle,  OE  is  found  to  be  408  volts.  This  being 
the  A.C.  volts,  division  by  the  ratio  0.71  will  give  the 
voltage  at  the  commutator.  Thus,  under  the  conditions 
assumed,  the  D.C.  voltage  at  no  load  is  determined  to  be 
575  volts. 

At  full  load  let  it  be  assumed  that  the  added  field 
strength  due  to  the  series  winding  is  slightly  more  than 
sufficient  to  bring  the  current  into  phase  with  the  E.M.F., 
and  that  in  fact  a  small  angle  of  lead  results,  say  five 
degrees.  The  full-load  diagram,  Fig.  123,  can  now  be 
drawn  as  follows: 

On  OE  draw  Ef  as  before,  only  now  instead  of  lagging 
77  degrees  the  current  is  leading  by  5  degrees.  Draw  EE0 
go  degrees  behind  El  and  equal  to  64  volts,  which  is  the 
E.M.F.  of  self-induction  at  full  load.  Draw  E0O  equal,  as 
before,  to  the  transformer  secondary  E.M.F.,  viz.,  426  volts. 
Then  OE  will  be  the  E.M.F.  delivered  to  the  collector  rings. 
With  the  values  as  chosen,  a  solution  of  the  triangle  OEE0 
will  give  OE  equal  to  OE0,  and  the  collector-ring  voltage 
will  thus  be  426  volts,  corresponding  to  a  direct-current 
pressure  of  600  volts. 

It  is  thus  seen  that  with  constant  E.M.F.  at  the  secondary 
terminals  of  the  step-down  transformers,  the  direct  current 
E.M.F.  of  the  converter  increases  from  575  volts  at  no  load 
to  600  volts  at  full  load,  an  increase  of  4.35  per  cent.  The 


ROTARY    CONVERTERS.  211 

condition  of  constant  secondary  transformer  potential  is, 
however,  seldom  met  in  practice  on  account  of  the  voltage 
drop  in  the  transmission  lines  due  to  the  load.  Hence, 
assuming  a  loss  of  4.35  per  cent  in  the  high-tension  feeders, 
the  secondary  voltage  of  the  transformers  will  be  4.35  per 
cent  lower  at  full  load  than  at  no  load;  and  this  fall  of 
potential  will  offset  the  increase  of  voltage  at  the  collector 
rings  which  would  otherwise  follow.  Therefore,  taking  into 
account  both  the  line  loss  and  the  control  of  voltage  through 
the  action  of  phase  displacement,  as  just  explained,  the  de- 
livered direct-current  potential  will  be  the  same  at  full  load 


/  -- 

Fig.  1230 

as  at  no  load,  the  condition  in  this  case  corresponding  to  the 
action  of  a  continuous-current  generator  compounded  for 
constant  voltage.  Similarly,  if  the  line  loss  is  greater  than 
can  be  compensated  for  by  the  phase  displacement,  the  vol- 
tage at  the  D.C.  brushes  will  fall  off  with  increase  of  load, 
while  if  the  line  loss  is  less,  a  small  over-compounding  is 
obtained. 

The  converter  having  supplied  to  it  a  given  voltage  on  the 
alternating  end  will  deliver  a  given  voltage  on  the  direct- 
current  end,  depending  on  the  practically  constant  and  fixed 
value  of  the  conversion  ratio;  and  the  variation  in  the 
delivered  D.  C.  pressure  is  due  not  to  any  addition  of  vol- 


212     POLYPHASE  APPARATUS  AND  SYSTEMS. 

tage  within  the  converter,  but  to  the  action  of  the  converter  in 
producing  a  difference  in  the  phase  displacement,  as  a 
result  of  which  a  change  is  brought  about  in  the  voltage 
which  is  delivered  to  the  converter.  It  is  thus  seen  that 
it  is  in  reality  not  the  converter  which  is  compounded,  but 
rather  the  system  as  a  whole. 

In  the  example  worked  out  above  the  calculation  was 
based  on  an  E.M.F.  of  self-induction  equivalent  to  15 
per  cent  of  the  normal  potential.  To  obtain  in  ordinary 
working  a  reasonable  range  of  voltage  control  without 
excessive  variations  in  power  factor,  converters  for  2  5 -cycle 
circuits  should  operate  on  a  reactance  of  from  1 7  to  20  per 
cent.  For  higher  frequencies  this  value  is  usually  some- 
what lower  on  account  of  possible  difficulties  from  pulsation 
which  are  aggravated  when  the  reactance  is  too  great.  The 
self-induction  of  lines,  transformers,  and  connections,  taken 
collectively  will  seldom  exceed  5  to  7  per  cent  under  normal 
conditions,  so  that  artificial  reactances  are  frequently  inserted 
sufficient  to  bring  the  total  up  to  the  desired  value.  These 
may  be  connected  at  any  point  in  the  circuit  between  genera- 
tor and  converter,  but  are  almost  invariably  connected  be- 
tween the  transformer  secondaries  and  the  collector  rings, 
where  they  give  the  advantage  of  individual  voltage  control 
to  converters  operated  in  parallel  and  where  their  windings 
are  subjected  to  only  moderate  potential  strains. 

Power  Factor.  —  Owing  to  the  small  and  practically 
negligible  armature  reaction,  there  will  be  little  change  in 
power  factor  with  change  of  load,  provided  that  the  field 
strength  is  kept  constant.  A  uniform,  though  adjustable, 
power  factor  is  therefore  the  characteristic  of  the  shunt- 
wound  type,  while  the  compound-wound  machine  with  its 
varying  excitation  gives  a  variable  power  factor,  the  amount 


ROTARY   CONVERTERS.  213 

of  variation  with  change  of  load  depending  on  the  relative 
strength  of  the  shunt  and  series  windings.  As  commonly 
adjusted,  compound  converters  will  give  a  power  factor  of 
unity  at  about  full  load  when  arranged  for  flat  compounding 
with  5  per  cent  line  drop  and  with  a  total  reactance  in 
circuit  of  1 8  per  cent.  Under  conditions  where  a  greater 
compounding  can  be  obtained,  the  series  field  will  be  ad- 
justed to  give  unity  power  factor  at  about  three-quarters 
load.  In  either  case  the  power  factor  at  light  load  will 
be  rather  low,  the  current  at  no  load,  as  shown  in  Fig.  122, 
being  almost  wholly  wattless.  A  variation  of  the  reactance 
in  the  supplying  circuit  will  change  the  values  of  the  excita- 
tion that  must  be  furnished  to  produce  unity  power  factor 
on  the  system  at  any  particular  load.  This  is  for  the 
reason  that  the  E.M.F.  of  self-induction  varies  with  the 
reactance,  and  on  it  depends  the  value  of  the  wattless 
lagging  component  of  the  main  line  current,  which  is  to  be 
neutralized  by  the  equal  and  opposite  wattless  (leading) 
component  which  the  converter  must  produce  if  unity  power 
factor  on  the  system  is  to  result. 

The  small  amount  of  armature  reaction  present  in 
rotary  converters  is  explained  by  the  fact  that  the  two  dis- 
tinct actions,  motor  action  and  generator  action,  which 
take  place  simultaneously  in  the  armature,  are  of  opposite 
sign  and  thus  virtually  neutralize  each  other.  The  net 
result  is  that  the  behavior  of  the  machine  considered  as  a 
converter  is  practically  that  of  a  machine  in  which  armature 
reaction  is  absent.  Considering  the  converter  as  a  genera- 
tor, the  armature  reaction  may  be,  and  frequently  is,  quite 
high,  higher  in  most  cases  than  would  be  advisable  in  a  con- 
tinuous-current generator  of  equal  capacity  and  speed. 
When  the  armature  reaction  of  a  converter  is  referred  to 


214     POLYPHASE  APPARATUS  AND  SYSTEMS. 

quantitively  it  is  the  armature  reaction  of  the  machine 
considered  as  a  continuous-current  generator  that  is  meant. 

Although  through  the  counteracting  influence  of  the 
two  opposing  currents  the  effective  reaction  is  made  almost 
nil,  there  yet  remains  a  small  residual,  namely,  that  due  to 
the  current  required  to  supply  the  losses  of  the  machine. 
Hence,  it  is  usually  desirable  to  give  the  direct-current 
brushes  a  slight  amount  of  shift  to  bring  them  into  the  best 
position  for  commutation.  When  the  converter  is  used  in 
the  ordinary  sense,  that  is,  to  convert  alternating  into  con- 
tinuous current,  the  direction  of  the  brush  shift  is  forward, 
or  in  the  direction  of  armature  rotation,  while  for  inverted 
operation  the  direction  of  shift  is  in  the  opposite  sense,  or 
against  the  direction  of  rotation. 

Limit  of  Frequency.  —  The  limit  of  frequency  of  a 
rotary  converter  is  determined  by  mechanical  considera- 
tions. In  designing  a  machine  for  a  given  number  of  alter- 
nations, the  problem  is  to  keep  the  peripheral  speed  of  the 
commutator  within  practical  limits.  Too  high  a  peripheral 
speed  will  cause  >  the  commutator  segments  to  buckle, 
through  the  action  of  centrifugal  force.  A  reduction  in 
the  diameter  of  the  commutator,  on  the  other  hand,  may 
reduce  the  width  of  the  segments  below  the  lowest  limit 
fixed  by  experience  with  commutator  construction  and 
operation.  The  voltage  and  output  will  determine  the 
general  dimensions  of  the  commutator.  Take  the  case  of 
a  6oo-kilowatt,  5  50- volt,  6o-cycle  rotary  converter,  the 
speed  of  which,  on  account  of  its  size,  is  limited  to,  say, 
600  revolutions  per  minute.  The  number  of  poles  would 
be  12.  The  peripheral  speed  of  the  commutator  being 
limited,  the  circumference  is  at  once  fixed.  The  average 
volts  per  bar  being  also  limited,  the  total  number  of  seg- 


ROTARY   CONVERTERS.  215 

ments  is  determined.  In  a  4o-cycle  rotary  recently  con- 
structed, the  average  voltage  between  segments  was  limited 
to  13!  volts,  and  the  commutator  speed  to  4500  feet  per 
minute.  If  we  apply  this  data  to  the  6o-cycle  converter, 
we  have  the  following: 

Number  of  segments  between  poles   =  550  ~-  13^  =  41. 
Total  number  of  segments  =  12  (number  of  poles)  X  41  =  492. 

That  circumference  of  the  commutator  which  will  keep 
the  peripheral  speed  within  the  limits  set  —  i.e.,  4500  feet 
per  minute  —  is  90  inches,  thus  allowing  only  0.18  inch  for 
the  width  of  each  segment.  For  mechanical  reasons  this 
width  is  less  than  can  be  used.  It  will  be  seen  that, 
unless  the  speed  can  be  increased,  thus  permitting  a 
smaller  number  of  poles,  or  the  peripheral  speed  of  the 
commutator  can  be  increased,  permitting  a  larger  circum- 
ference, and  consequently  wider  segments,  the  difficulty 
can  be  overcome  only  by  using  two  commutators,  each 
delivering  275  volts  and  connected  in  series  for  550  volts. 
This,  however,  involves  a  complication  of  collector  rings 
and  connections,  and  the  current  must  be  commutated 
twice  and  the  commutator  losses  doubled.  This  con- 
verter could  be  built  readily  with  one  commutator  if  .wound 
for  250  volts  or  thereabouts.  The  general  statement  may 
be  made  that  for  frequencies  over  35  to  40  cycles  it  is  more 
difficult  to  build  converters  for  high  voltage  than  for  low 
voltage  —  i.e.,  for,  say,  600  volts,  than  for  100  to  200 
volts  —  but  not  such  a  difficult  problem  to  wind  a  con- 
verter of  less  than  35  cycles  for  the  higher  voltage. 

The  following  table  shows  the  speed  and  approximate 
weight  of  railway  service  converters  of  a  standard  make  for 
various  outputs  and  for  frequencies  of  25  and  60  cycles: 


216 


POLYPHASE   APPARATUS   AND    SYSTEMS. 
25-CvcLE  CONVERTERS. 


Poles. 

Capacity 
K.W. 

Speed 
R.P.M. 

Weight 
Pounds. 

4 

200 

75° 

15,000 

4 

300 

75° 

20,500 

6 

4OO 

500 

30,000 

6 

500 

500 

35>°°° 

6 

75° 

500 

45,000 

8 

1,000 

375 

58,000 

12 

1,500 

250 

92,000 

6o-CvcLE  CONVERTERS. 


6 

IOO 

1,200 

7,000 

6 

2OO 

I,2OO 

11,000 

8 

300 

900 

18,000 

12 

500 

600 

33,000 

Parallel  Operation.  —  The  conditions  of  service  require 
that  converters  should  be  able  to  work  in  parallel  with  one 
another,  and  they  can  be  so  operated  with  entire  facility. 
To  insure  proper  division  of  the  load  each  converter  should 
obviously  deliver  the  same  direct-current  voltage.  This 
demands  either  that  the  conversion  ratios  of  all  the  machines 
should  be  the  same  or  that  the  voltage  delivered  to  the  col- 
lector rings  should  be  such  that  any  difference  of  ratio  will 
be  compensated  for.  The  usual  arrangement  is  to  feed 
each  converter  from  its  own  bank  of  transformers.  This 
plan  saves  the  cost  and  complication  of  low  tension  A.C. 
bus  bars  and  dispenses  with  switches  and  measuring  in- 
struments in  the  low-tension  circuit.  It  has  the  further 
advantage  that  it  permits  some  independent  voltage  adjust- 
ment to  be  given  to  each  converter,  through  the  fact  that  a 
phase  displacement  following  a  change  in  excitation  acts  on 
the  self-induction  of  a  local  and  independent  circuit.  The 


ROTARY    CONVERTERS.  217 

range  of  individual  voltage  adjustment  is  greatest  for  a 
condition  of  maximum  self-induction,  corresponding  to  the 
use  of  the  artificial  reactance  supplied  for  compounding. 
Under  this  condition  it  is  frequently  possible  to  equalize  the 
D.C.  voltage  and  the  load  even  where  the  conversion  ratio 
or  the  ratio  of  the  step-down  transformers,  is  several  per 
cent  off. 

Starting  of  Rotary  Converters.  —  Rotary  converters  may 
be  started  by  auxiliary  motors  after  the  manner  already 
described  for  synchronous  motors.  In  this  case  the  con- 
verter, after  being  excited,  has  to  be  synchronized  before 
being  connected  to  the  alternating-current  supply. 

Converters  may  also  be  started  from  the  direct-current 
end,  in  the  same  way  as  a  direct-current  shunt  or  com- 
pound-wound motor.  This  mode  of  starting  also  requires 
the  machine  to  be  synchronized,  the  necessary  adjust- 
ment of  speed  being  accomplished  by  variation  of  the 
shunt-field  strength. 

The  objections  to  methods  of  starting  which  require  syn- 
chronizing are  that  considerable  time  may  be  required  for 
getting  the  machine  in  service,  particularly  where,  as  is  fre- 
quent in  railway  work,  the  line  voltage  or  frequency  is 
unsteady,  due  to  fluctuations  in  the  load.  Unless  it  is 
important  to  keep  the  starting  current  within  the  lowest 
possible  limits,  therefore,  the  most  convenient  method  of 
starting  is  to  apply  alternating  voltage  direct  to  the  col- 
lector rings  of  the  converter,  the  converter  thus  starting  like 
an  induction  motor.  Under  these  conditions  the  starting 
torque  is  made  reasonably  good  by  the  use  of  solid  poles,  or 
if  the  poles  are  laminated,  by  the  use  of  bridges  or  a  short- 
circuited  "amortisseur"  winding.  One  or  the  other  of 
these  constructions  is  usually  employed  in  any  event,  as 


218 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


their  tendency  is  to  minimize  troubles  from  hunting  and  to 
improve  the  stability. 

The  voltage  to  be  applied  at  starting  may  be  reduced 
by  a  compensator,  or,  since  transformers  are  nearly  always 
used  with  rotary  converters,  by  the  use  of  fractional  voltage 
taps  in  the  secondary  winding.  With  converters  of  400 
kilowatts  and  under  a  single  tap  at  half  voltage  is  usually 
provided,  while  with  converters  of  over  400  kilowatts  two 
taps  are  advisable,  at  i  and  f  voltage.  One  or  two  double- 


VWWWW        VWWWW        VWvWVW 


Fig.  124. 

throw  switches,  dependent  upon  the  number  of  taps  brought 
out,  are  used,  as  shown  in  Figs.  124  and  125.  In  Fig.  124, 
showing  the  connections  of  a  three-phase  converter,  its 
transformers  having  half  voltage  taps,  a  double- pole,  double- 
throw  switch  is  used.  Referring  to  the  diagram,  when  the 
switch  is  in  the  "up"  position,  half  voltage  is  impressed  on 
the  collector  rings,  and  when  the  switch  is  thrown  down  full 
voltage  is  impressed.  In  Fig.  125,  which  shows  the  connec- 
tions of  a  six-phase  machine  with  transformers  having  J  and 
f  taps,  two  triple-pole,  double-throw  switches  are  used. 
In  starting,  both  switches  are  thrown  to  the  "up"  position, 


ROTARY   CONVERTERS. 


219 


thus  applying  J  voltage.  The  next  step  is  to  throw  the 
left-hand  switch  to  the  "down"  position,  which  applies  f 
voltage.  Finally,  the  right-hand  switch  is  thrown  to  the 
"down"  position,  which  impresses  full  potential  across  the 
collector  rings. 

Well  designed  converters  when  started  from  the  alter- 
nating-current side  in  the  manner  described,  will  draw  not 
more  than  about  full-load  current  from  the  line.  Where 


converters  are  started  in  this  way  there  is  an  equal  chance 
that  the  polarity  of  the  direct-current  brushes  will  be  right 
or  wrong.  In  the  event  of  the  converter's  locking  itself 
into  step  with  the  direct-current  potential  reversed,  the 
polarity  may  be  corrected  by  separately  exciting  the  fields 
of  the  machine  from  the  bus  bars.  This  should  be  done 
while  the  collector  rings  are  receiving  their  current  from 
the  low-voltage  tap  on  the  transformer.  This  is  more  con- 
veniently accomplished,  however,  by  the  use  of  a  double- 


22O     POLYPHASE  APPARATUS  AND  SYSTEMS. 

throw  type  of  field  switch  in  the  self-exciting  circuit,  so  that 
from  the  terminals  of  the  machine  itself  current  may  be  sent 
through  the  fields  in  either  direction.  In  this  way  a  change 
cf  polarity  can  be  effected  without  recourse  to  an  external 
source  of  current.  Where  this  switch  is  called  into  play  it 
should  also  be  used  while  the  converter  is  running  at  low 
voltage,  since  an  attempt  to  reverse  the  field  at  full  voltage 
not  only  causes  bad  sparking  at  the  commutator  but  would 
probably  prove  unsuccessful,  the  reason  being  that  the 
synchronizing  power  with  full  alternating  voltage  applied, 
due  to  the  induced  field  set  up  by  the  armature  current,  is 
too  strong  to  permit  the  relative  retrograde  movement  of  the 
armature  that  takes  place  on  reversal  of  polarity. 

The  field  switch,  whether  of  the  double-throw  type  or 
not,  should  be  made  with  several  blades  and  contacts,  so 
that  when  open  the  field  circuit  is  divided  into  sections,  as 
shown  in  Fig.  84,  in  order  to  minimize  the  voltage  induced 
in  the  field  winding  at  starting. 

Equalizers  must  always  be  used  where  compound- wound 
converters  are  run  in  parallel,  as  is  usual  with  compound- 
wound,  continuous- current  generators. 

A  compound-wound  converter,  when  operating  in  paral- 
lel with  other  machines,  will  take  current  from  the  bus  bars 
and  run  as  a  direct-current  motor  if  its  voltage  is  low,  or  if 
the  alternating-current  side  is  disconnected  from  the  A.C. 
supply,  or  in  the  event  of  a  short  circuit  in  the  alternating- 
current  leads  or  windings.  The  direction  in  which  the 
series  coils  are  wound,  while  adding  to  the  field  strength 
when  the  machine  is  delivering  current,  will  act  in  opposition 
to  the  excitation  of  the  shunt  coils  when  current  is  flowing 
into  the  machine;  and  if  the  machine  has  a  strong  series 
winding  it  is  likely  therefore  to  increase  greatly  in  speed 


ROTARY   CONVERTERS.  221 

when  running  in  this  way  as  a  direct-current  motor.  To 
guard  against  this,  compound  converters  are  desirably  pro- 
vided with  some  form  of  centrifugal  device  which,  by  clos- 
ing an  auxiliary  circuit,  will  trip  the  direct-current  circuit 
breaker  at  a  predetermined  increase  of  speed. 

Rotary  converters  are  also  commonly  provided  with  a 
small,  mechanical  oscillator,  or  end-play  device,  acting  on 
the  end  of  the  shaft,  to  give  the  shaft  a  small  reciprocating 
motion  in  the  bearings.  The  action  of  this  device  prevents 
the  rubbing  parts,  such  as  commutator,  collector  rings,  and 
bearings,  from  wearing  in  ridges  or  grooves,  which  would  be 
liable  to  form  in  the  absence  of  any  end- play. 

Hunting.  —  Hunting  of  rotary  converters  is  caused  by 
the  same  conditions  that  similarly  affect  synchronous 
motors,  and  is  corrected  by  similar  expedients.  Variation 
in  the  turning  moment  of  the  prime  movers,  short  circuits, 
sudden  changes  of  load,  too  high  resistance  in  the  trans- 
mission lines,  defective  design  of  the  converters,  are  all 
factors  which  affect  the  stability.  The  hunting  tendency 
naturally  increases  with  the  frequency.  At  25  cycles 
stability  may  be  readily  obtained.  The  problem  is  rather 
more  difficult  at  60  cycles,  and  requires  skillful  attention  and 
careful  adjustment  to  local  conditions. 

Inverted  Converters.  —  When  running  inverted,  that  is 
when  converting  from  continuous  to  alternating  current, 
the  speed  of  the  converter,  as  in  a  direct-current  motor, 
depends  upon  the  field  strength,  being  increased  by  a  weak 
field  and  decreased  by  a  strong  field.  A  change  in  the 
setting  of  the  field  rheostat  therefore  affects  only  the 
delivered  frequency  and  not  the  voltage,  the  A.C.  voltage 
being  determined  only  by  the  product  of  the  direct-current 
voltage  into  the  conversion  ratio  of  the  machine.  A  con- 


222     POLYPHASE  APPARATUS  AND  SYSTEMS. 

verter  intended  to  run  inverted  should  have  little  or  no 
series  field,  or  it  will  change  in  speed  with  variations  of  load 
and  thus  deliver  an  unsteady  frequency.  If  an  inverted 
converter  is  supplying  current  to  an  inductive  circuit,  the 
lagging  current  delivered,  which  is  demagnetizing  in  its 
action,  tends  to  weaken  the  field  strength  and  hence  to  in- 
crease the  speed.  Under  extreme  conditions,  the  reduc- 
tion in  field  strength  may  be  sufficient  to  cause  an  excessive 
rise  in  speed.  An  efficient  form  of  speed  limiting  device 
is  therefore  especially  important  for  converters  when  run 
inverted. 

Double-Current  Generators.  *•-  This  term  is  applied  to 
generators  designed  to  give  either  alternating  or  continuous 
current,  or  both  alternating  and  continuous  current  at  the 
same  time.  They  may  be  wound  for  any  desired  number 
of  phases  on  the  alternating  end  and  on  the  direct-current 
end  for  any  voltage  within  the  limits  of  continuous-current 
design,  the  ratio  between  the  alternating  and  the  direct- 
current  voltage  being  essentially  the  same  as  in  a  converter 
of  the  same  number  of  phases.  They  have  the  general 
appearance  of  rotary  converters,  being  equipped  with  a 
commutator  on  the  direct- current  end  and  with  collector 
rings  on  the  alternating-current  end,  but  have  in  addition 
the  necessary  mechanical  parts,  such  as  are  provided  with 
generators,  to  adapt  them  for  mechanical  driving.  They 
are  most  conveniently  constructed  for  fairly  high  speeds 
because  if  built  for  a  low  speed  the  large  number  of  poles 
that  is  necessary  to  bring  the  frequency  within  commercial 
limits  results,  for  ordinary  sizes,  in  a  very  thin  or  narrow 
armature  and  an  uneconomical  construction. 

Double-current  generators  are  designed  with  much  lower 
armature  reaction  than  rotary  converters.  The  field  cir- 


ROTARY   CONVERTERS.  223 

cuit  may  be  designed  for  excitation  from  the  direct-current 
brushes,  but  where  a  considerable  proportion  of  the  out- 
put is  to  be  delivered  in  the  form  of  alternating  current  the 
reaction  on  the  field  under  conditions  of  inductive  load 
lowers  the  direct  current  E.M.F.  This  in  turn  reduces  the 
excitation  and  magnifies  the  fall  of  potential  with  increases 
of  load.  For  these  reasons  separate  excitation  is  usually 
desirable. 


224     POLYPHASE  APPARATUS  AND  SYSTEMS. 


CHAPTER   IX. 

MOTOR  GENERATORS,  FREQUENCY  CHANGERS, 
AND  OTHER  CONVERTING  APPARATUS. 

IN  the  conversion  of  electrical  energy  from  one  voltage 
or  frequency  into  some  other  voltage  or  frequency,  either 
a  single  apparatus  may  be  used  specifically  adapted  to 
the  purpose,  or  a  combination  of  two  or  more  machines  or 
pieces  of  apparatus  may  be  employed.  For  converting 
alternating  into  continuous  current,  for  example,  a  rotary 
converter  *  may  be  used,  or  the  conversion  may  be  effected 
by  the  medium  of  an  apparatus  consisting  of  a  continuous- 
current  generator  driven  by  a  suitable  alternating-current 
motor.  For  other  classes  of  work  the  generator  may  be 
of  the  alternating-current  type,  or  the  motor  may  be  wound 
for  continuous  current;  or  both  motor  and  generator  may 
be  of  the  same  type.  Such  an  apparatus  is  called  a  motor 
generator. 

The  two  elements,  motor  and  generator,  are  usually 
coupled  mechanically  together  and  mounted  on  a  common 
bed-plate.  In  alternating-current  work  either  a  syn- 
chronous or  an  induction  motor  may  be  employed  for 
the  motor  element.  The  modern  tendency  in  large  units 
is  toward  synchronous  motors,  both  because  of  their 
advantage  in  the  matter  of  phase  control  and  because 
they  can  be  more  readily  wound  for  high-voltage  supply 
circuits. 

*  See  chapter  on  Rotary  Converters. 


MOTOR   GENERATORS,  ETC.  225 

Motor    Generators    Supplying    Continuous    Current.  — 

Where  the  motor  element  in  a  set  delivering  continuous 
current  is  of  the  synchronous  type,  the  necessary  field 
excitation  may  be  supplied  from  the  direct-current  gener- 
ator, provided  the  voltage  of  this  is  not  ^tdo^high  (see 
remarks  on  excitation  potential  on  page  27);  otherwise 
the  field  circuit  is  supplied  from  an  independent  exciter, 
which  is  frequently  direct  connected.  Starting  and  syn- 


Fig.  126. 

chronizing  of  synchronous  sets  is  effected  by  the  same 
methods  as  have  been  described  in  Chapter  VI.  for  syn- 
chronous motors ;  or  where  a  direct-current  supply  is  avail- 
able either  from  other  sets  in  operation  or  from  a  storage 
battery,  the  motor-generator  set  may  be  started  from  the 
direct-current  end,  using  the  generator  element  as  a  motor. 
The  general  appearance  of  a  two-unit  set,  consisting  of 
continuous-current  generator,  synchronous  motor  and 
direct-connected  starting  motor,  is  shown  in  Fig.  126. 
This  set  has  a  normal  output  of  400  kilowatts  at  600  volts 


226 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


on  the  continuous-current  side,  and  embodies  the  standard 
construction  of  the  Westinghouse  Company. 

A  motor  generator  built  by  the  Allgemeine  Company  is 
illustrated  in  Fig.  127.  The  driving  motor  is  of  the  induc- 
tion type,  and  the  direct-connected  exciter  provides  a 
variable  excitation  to  the  continuous-current  generator, 
which  works  on  the  Ward-Leonard  system.  This  is  the 


Fig.  127. 

type  of  motor  generator  used  in  the  Ilgner  system,  the  fly 
wheel  (not  shown  in  the  cut)  being  carried  in  separate 
bearings  at  the  motor  end  of  the  set. 

The  advantage  of  the  motor  generator  over  the  rotary 
converter  lies  principally  in  the  fact  that  the  voltage  on  the 
continuous-current  end  is  independent  of  voltage  fluctua- 
tions on  the  alternating-current  end.  The  motor  and  gen- 
erator windings  being  electrically  distinct,  the  generator 
voltage  for  a  given  condition  of  load  and  excitation  depends 


MOTOR   GENERATORS,  ETC.  22/ 

only  on  the  speed,  which  is  not  affected  by  changes  in  the 
alternating  voltage  except  to  a  limited  extent  where  the 
motor  is  of  the  induction  type.  The  generator  may  be 
compounded  for  automatic  voltage  regulation  to  a  degree 
that  would  not  be  feasible  in  a  rotary  converter.  A  further 
advantage,  where  the  driving  motor  is  of  the  synchronous 
type,  is  the  possibility  of  greater  compensation  by  phase 
control  for  lagging  current  on  other  parts  of  the  system. 
The  motor  may  also  be  wound  direct  for  line  potential 
where  this  is  not  too  high,  thus  saving  the  cost  of  step- 
down  transformers.  The  motor  generator  is  especially 
useful  on  circuits  of  60  cycles  and  over,  particularly  in 
large  capacities  where  the  design  of  a  rotary  converter 
might  be  difficult. 

As  an  offset  to  these  advantages  the  motor  generator  is 
inferior  in  efficiency,  higher  in  cost,  and  usually  requires 
more  floor  space.  Taking  the  efficiency  of  the  motor  and 
generator  at  93  and  92  per  cent  respectively,  the  total 
efficiency  of  conversion  is  85.6  per  cent.  The  conversion 
efficiency  with  a  rotary  converter  of  equal  output,  including 
the  transformers  that  are  usually  required,  would  be  the 
product  of  converter  efficiency,  say  94  per  cent,  and  of 
transformer  efficiency,  say  97.5  per  cent,  or  91.7  per  cent 
over  all,  showing  a  better  performance  by  about  6  per  cent. 
From  the  standpoint  of  first  cost,  and  assuming  the  same 
speed  in  both  cases,  the  rotary  converter  with  its  trans- 
formers will  require  the  smaller  investment;  for  even  if 
the  cost  of  the  generator  is  considered  to  be  balanced  by 
that  of  the  converter,  it  is  obvious  that  the  cost  of  the 
motor  will  exceed  that  of  the  transformers.  The  difference 
is  lessened  when  the  motor-generator  set  can  be  designed 
for  a  higher  speed  than  the  rotary  converter.  The  saving 


228     POLYPHASE  APPARATUS  AND  SYSTEMS. 

by  the  use  of  three-phase  transformers  for  the  converter 
may  offset  this  gain.  The  net  difference  generally  will  be 
found  to  be  from  10  to  30  per  cent  or  more  against  the 
motor  generator. 

In  the  matter  of  floor  space  the  difference  need  not  be 
very  marked  if  the  motor  generator  is  of  high  speed,  and 
if  reference  is  had  not  merely  to  the  net  but  to  the  gross 
floor  space,  by  which  is  meant  the  actual  ground  area 
covered  by  the  apparatus,  plus  an  allowance  for  the 
necessary  passageways. 

Rotary  converters,  which  were  first  developed  in 
America,  have  enjoyed  a  wider  use  in  this  country  than  in 
Europe,  where  the  motor  generator  is  more  generally 
employed.  Experience  has  shown  that  each  type  has  its 
own  peculiar  advantages,  and  a  greater  discrimination  than 
formerly  is  now  being  exercised  by  engineers  in  making  a 
choice  between  the  two. 

Motor  Generators  as  Frequency  Changers.  —  In  alter- 
nating-current plants  employing  a  low  frequency,  there  is 
sometimes  a  need  for  a  limited  amount  of  current  of  a 
higher  frequency.  For  instance,  a  supply  of  current  for 
incandescent  or  arc  lighting  may  be  required  from  an 
installation  where  the  frequency  is  25  cycles.  To  meet 
such  cases  a  frequency  of  60  cycles,  or  any  other  period- 
icity suitable  for  lighting,  may  be  obtained  from  a  motor- 
generator  set  in  which  a  generator  of  the  desired  output 
and  frequency  is  driven  by  a  motor  taking  its  supply  from 
the  low-frequency  circuit.  In  such  cases  the  motor  is 
usually  of  the  synchronous  type,  which  insures  uniform 
frequency  in  the  generator  independent  of  load  fluctua- 
tions, and  also  makes  it  possible  to  operate  the  motor 
generator  in  the  reverse  sense,  i.e.,  taking  power  from  the 


MOTOR   GENERATORS,  ETC. 

high-frequency  mains  and  delivering  energy  at  the  low 
frequency.  This  feature  is  valuable  in  the  not  uncommon 
case  where  such  frequency-changing  sets  are  used  to  tie 
together  two  separate  systems  of  different  frequency  which 
may  supply  the  same  or  adjacent  territory. 

Since  the  two  machines  constituting  the  frequency- 
changing  set  are  mechanically  coupled,  the  speed  for 
which  the  set  is  designed  must  be  one  which  will  give 
correct  frequency  in  both  motor  and  generator  elements 
simultaneously,  having  regard  to  the  number  of  poles 
with  which  each  member  is  provided.  Where  the  ratio  of 
the  higher  to  the  lower  frequency  is  a  whole  number,  as 
in  a  set  transforming  from  25  to  50  cycles,  this  requirement 
is  easily  fulfilled.  In  the  case  assumed,  the  generator 
would  have  twice  as  many  poles  as  the  motor,  and  the  set 
could  be  designed  for  any  speed  which  would  suit  the 
motor,  according  to  the  equation 

1 20  X  Frequency 
No.  of  poles 

The  choice  of  speed  is  more  restricted  where  the  ratio 
of  frequencies  is  not  a  simple  number.  Taking  the  case 
of  a  conversion  from  25  to  60  cycles,  the  possible  speeds 
for  the  25-cycle  motor  are  as  follows: 

Motor,  25  Cycles. 

POLES  SPEED 

2  1500 

4  75° 

6  500 

8  375 

10  300 

etc. 


23O     POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  speeds  nearest  to  the  above  values  and  correspond- 
ing to  a  frequency  of  60  cycles  are : 

60  Cycle  Generator. 

POLES  SPEED 

6  1200 

10          720 

14  5U 

1 8          400 

20  360 

24  300 

etc. 

Considerations  of  first  cost  point  to  the  choice  of  a  high- 
speed design,  yet  in  this  case  the  highest  available  speed 
that  is  common  to  both  frequencies  is  300  revolutions,  a 
speed  which  is  very  low  for  self-contained  apparatus  of 
this  description.  Where  it  is  unnecessary  to  preserve  the 
exact  ratio  of  frequency,  a  compromise  can  be  effected,  as 
by  selecting  a  four-pole  design  for  the  motor  giving  a  speed 
of  750  revolutions  and  a  delivered  frequency  of  62.5  cycles 
with  a  ten-pole  generator. 

In  the  parallel  operation  of  frequency-changing  sets  an 
equal  division  of  load  cannot  be  secured  unless  the  rela- 
tive angular  position  of  the  rotating  elements  of  motor 
and  generator  respectively  is  the  same  in  each  set.  If 
in  one  set  the  angular  position  of  the  generator  is  leading 
with  reference  to  that  of  the  other  machines,  it  will  take 
more  than  its  share  of  load,  and  vice  versa  if  lagging. 
Accurate  machine  work  is  therefore  necessary  in  fixing  on 
the  shaft  the  relative  angular  position  of  the  two  revolving 
elements  of  the  set,  although  even  with  the  utmost  care  it 
is  not  possible  to  locate  these  parts  with  absolute  accuracy. 
The  equivalent  result  is  secured  in  machines  of  recent 


MOTOR   GENERATORS,   ETC. 


231 


manufacture  by  so  arranging  the  stationary  element  of  one 
of  the  two  machines  that  it  can  be  given  a  small  angular 
shift.  This  scheme  is  embodied  in  the  frequency-changing 
set  shown  in  Fig.  128,  which  is  one  of  500  kilowatts  capa- 
city converting  from  25  cycles  at  13,000  volts  to  62.5  cycles 
at  4000  volts.  The  circular  structure  constituting  the 
stationary  armature  of  the  generator  is  carried  in  a  cradle 
formed  by  the  two  supporting  feet.  By  the  two  vertical 


Fig.  128. 


set  screws  seen  on  the  outside  of  the  frame  the  necessary 
angular  adjustment  is  made,  after  which  the  frame  is 
clamped  in  place.  The  adjustment  may  be  made  on  the 
motor  with  the  same  result. 

Induction  Type  Frequency  Changer.  —  In  another  type 
of  apparatus  for  changing  the  frequency,  the  generator 
element  is  substantially  like  an  induction  motor  with  polar- 
wound  armature,  the  armature  being  rotated  by  a  syn- 


232     POLYPHASE  APPARATUS  AND  SYSTEMS. 

chronous  motor  in  a  direction,  for  increase  of  frequency, 
opposite  to  that  in  which  it  would  naturally  tend  to  revolve. 
The  low-frequency  current  is  fed  to  the  primary  or  field, 
and  the  high-frequency  current  is  taken  from  the  secondary 
or  armature  by  means  of  collector  rings.  The  frequency 
of  the  output  will  depend  on  the  speed  and  direction  of 
rotation  of  the  secondary  and  on  the  number  of  poles  for 
which  the  primary  is  wound. 

If     N1  =  natural    speed    corresponding    to    number    of 
poles  in  primary  at  the  frequency  impressed, 

and  if  N2  =  forced  speed  of  rotation,  N2  being  negative 
for  rotation  in  the  natural  sense  and  positive 
for  rotation  in  the  opposite  sense,  the  frequency 
delivered  by  the  secondary  will  be 

N  +N 

Secondary  Frequency  =  — VT — 2X  Primary  Frequency. 

•"i 

Thus,  if  the  secondary  is  run  at  natural  speed  but  in 
opposition  to  its  natural  direction  of  rotation,  the  secondary 
frequency  will  be  double  that  of  the  primary.  If  run  at 
half  natural  speed  in  the  natural  direction,  the  secondary 
frequency  will  be  half  the  primary. 

To  convert  from  40  to  60  cycles,  we  could  use  a  four- 
pole  generator  element,  and  drive  the  secondary  at  600 
revolutions  per  minute  against  the  natural  direction  by 
means  of  an  eight-pole  motor.  The  natural  speed  of  the 
secondary  would  be  1200  revolutions.  By  driving  it  in 
the  opposite  direction  at  a  speed  of  600  revolutions,  the 
number  of  alternations  will  be  that  due  to  an  equivalent 


MOTOR   GENERATORS,  ETC.  233 

speed  of  1800  revolutions  in  a  four-pole  field,  or  60  cycles. 
Or,  by  the  above  formula, 

1200  +  600 
40  X—  —  =  60  cycles. 

1200 

The  capacity  of  the  driving  motor  bears  the  same 
proportion  to  the  total  output  that  the  increase  in 
frequency  bears  to  the  final  frequency.  In  the  generator 
element  the  capacity  of  the  secondary  must  equal  the 
output.  The  capacity  of  the  primary  has  the  same  pro- 
portion to  the  total  output  that  the  initial  frequency  bears 
to  the  final  frequency.  As  an  illustration,  the  above 
frequency  changer  when  intended  to  deliver  an  output  of 
100  kilowatts  would  be  made  up  as  follows,  neglecting 
losses : 

Generator  Element. 

Capacity  of  secondary  =100  K.W. 

40 
Capacity  of  primary     j-Xioo=   66.6  K.W. 


Motor  Element. 

Capacity  20 

^Xioo=   33.3  K.W. 

For  a  given  amount  of  energy  transformed,  this  type 
requires  a  minimum  of  capacity  in  the  transforming  unit. 
This  saving  is  largely  offset  by  certain  disadvantages, 
among  which  are  the  interdependence  of  the  windings, 
which  causes  voltage  fluctuations  on  the  primary  to 
reappear  equally  in  the  secondary;  the  absence  of  any 


234 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


simple  means  for  controlling  the  voltage  of  the  secondary; 
and  the  difficulty  of  winding  the  generator  for  high  voltage, 
either  on  armature  or  field.  These  have  caused  this  type 
of  machine  to  be  practically  superseded  by  the  motor 
generator. 

Motor  Converter.  —  This  device,  in  its  design  and  opera- 
tion, partakes  of  the  characteristics  both  of  the  rotary 
converter  and  the  motor  generator,  and  in  external  appear- 
ance is  not  essentially  different  from  the  latter. 

The  motor  converter  consists  of  two  elements,  an  induc- 


Fig,  129, 

tion  motor  and  a  rotary  converter,  which  are  mechanically 
coupled,  and  which  are  also  electrically  coupled  by  perma- 
nently connecting  the  rotor  of  the  motor  to  the  armature 
of  the  converter,  as  shown  on  the  accompanying  diagram 
of  a  three-phase,  bipolar  unit  (see  Fig.  129).  By  this 
arrangement  the  power  is  transmitted  through  the  induc- 
tion motor  element  of  the  machine,  partly  mechanically 
and  partly  electrically,  to  the  converter  element.  Inas- 
much as  the  motor  and  converter  are  connected  in  concat- 
enation, the  .  synchronous  speed  is  divided,  so  to  speak, 
between  these  two  elements  in  the  ratio  of  the  number 
of  poles  of  the  converter  element  to  the  total  number  of 


MOTOR  GENERATORS,  ETC.  235 

poles  of  both  alternating-current  and  continuous-current 
elements.  The  actual  shaft  speed,  therefore,  must  be  that 
corresponding  to  the  synchronous  speed  for  the  sum  of 
the  number  of  poles  on  both  machines,  and  hence  much 
lower  than  the  speed  of  an  ordinary  rotary  converter 
designed  for  the  same  frequency.  For  instance,  if  the  two 
elements  have  an  equal  number  of  poles  —  six  poles  on 
the  induction  motor  and  six  poles  on  the  converter  —  the 
actual  speed  of  the  machine  will  be  one-half  of  the  syn- 
chronous speed  of  an  ordinary  six-pole  rotary  converter 
supplied  from  the  same  alternating-current  line. 

Since  the  continuous-current  armature  of  the  motor  con- 
verter operates  partly  as  a  rotary  converter  armature  and 
partly  as  the  armature  of  a  continuous-current  generator, 
the  PR  loss  in  the  armature  conductors  and  the  field  loss 
due  to  armature  reaction  will  have  values  lying  between 
the  corresponding  losses  in  a  rotary  converter  and  in  a 
continuous-current  generator.  The  efficiencies  will  there- 
fore be  somewhat  higher  than  for  the  generator  of  the 
same  capacity  in  a  motor-generator  set  and  lower  than  for 
a  rotary  converter. 

Where  the  alternating  voltage  is  not  too  high,  the  motor 
element  may  be  wound  direct  for  line  potential  without 
step-down  transformers,  thereby  effecting  some  saving  in 
space  and  cost  on  the  complete  outfit.  Moreover,  the 
motor  converter  does  not  require  any  reactance  for  com- 
pounding, as  there  is  enough  reactance  for  this  purpose  in 
the  induction  motor.  Another  advantage  resides  in  the 
fact  that  main  slip  rings  are  done  away  with,  the  alter- 
nating-current rotor  and  the  continuous-current  armature 
being  simply  connected  together  along  the  shaft. 

One  of  the  chief  advantages  of  the  motor  converter  for 


236     POLYPHASE  APPARATUS  AND  SYSTEMS. 

the  higher  frequencies  is  that  a  much  lower  frequency  is 
impressed  on  the  converter  armature  than  is  supplied  from 
the  line  to  the  induction  motor.  This  feature  means  a 
converter  element  with  a  comparatively  small  number  of 
poles,  and  correspondingly  small  number  of  sets  of  brushes 
and  low  peripheral  speed  of  commutator,  resulting  in 
low  maintenance  and  good  commutation  characteristics. 
Since  the  dimensions  of  the  induction  motor  element 
depend  largely  upon  the  primary  frequency  and  not  upon 
the  speed  of  the  rotor,  considerable  weight  economy  will 
be  effected -in  motor  converters  for  the  higher  frequencies, 
as  compared  with  the  motor  generator. 

The  motor  converter  is  readily  started  from  the  alter- 
nating-current side  by  the  methods  customary  with  induc- 
tion motors  of  the  resistance-in-armature  type.  In 
parallel  operation,  voltage  control  and  other  characteris- 
tics, the  behavior  of  the  machine  is  analogous  to  that  of  a 
rotary  converter  with  considerable  reactance  in  circuit. 
The  motor  converter,  being  a  synchronous  piece  of  appa- 
ratus, is  subject  to  the  same  troubles  from  hunting  as 
those  to  which  rotary  converters  are  liable,  and  from  the 
same  causes,  —  which  may  be  treated  by  the  expedients 
employed  in  the  case  of  synchronous  apparatus  generally. 

Synchronous  Rectifier.  —  A  rectifier  is  a  device  for 
changing  an  alternating  current  into  a  continuous  current, 
without  the  aid  of  rotation  in  a  magnetic  field.  In  the 
single-phase  synchronous  rectifier  a  two-part  commutator 
is  revolved  in  synchronism  with  the  alternating-current 
supply,  the  two  halves  of  the  commutator  being  perma- 
nently connected  to  the  supply  circuit.  Two  opposite 
brushes  bearing  on  this  commutator  wrill  therefore  receive 
current  that  is  unidirectional  though  pulsating.  Polyphase 


MOTOR   GENERATORS,  ETC. 


237 


synchronous  rectifiers  are  constructed  on  similar  principles. 
Excessive  sparking  marks  the  operation  of  practically  all 
synchronous  rectifiers  unless  the  rectified  current  is  of  vqry 
small  value.  They  have  therefore  found  only  a, restricted 
application,  and  have  not  been  successfully  built  for  an. 
output  of  more  than  a  few  amperes. 

Mercury  Rectifier.  —  In  this  device,  which  embodies  no 
moving  parts,  application  is  made  of  that  property  pos- 
sessed by  ionized  mercury  vapor 
of  being  a  conductor  to  current  of 
one  polarity  only.  The  rectifier 
proper  consists  of  an  exhausted 
glass  vessel  or  tube  of  approxi- 
mately pear-shaped  form  (Fig. 
130).  Sealed  into  it  are  four 
terminals,  the  cathode,  B,  the  two 
anodes,  AA>  and  the  starting 
anode,  C.  The  two  anodes  are 
connected  across  the  alternating 
supply,  and  each  thus  becomes 
alternately  positive  and  negative 
during  the  successive  cycles.  The 
cathode,  located  at  the  bottom  of  the  tube,  is  covered  with 
mercury,  from  which  is  liberated  the  ionized  vapor  on 
whose  properties  as  a  conductor  to  currents  of  one  polarity 
the  working  of  the  device  depends.  When  either  anode  is 
positive,  the  mercury  vapor  within  the  tube  permits  an  arc 
to  be  formed  between  that  anode  and  the  cathode,  which 
is  negative.  When  the  polarity  of  the  alternating  current 
reverses,  the  other  anode  becomes  positive,  and  the  arc 
now  passes  from  the  second  anode  to  the  cathode,  which, 
is  still  negative,  no  current  flowing  from  the  first  anode, 


238    POLYPHASE  APPARATUS  AND  SYSTEMS. 

since  its  polarity  is  now  such  that  any  current  flow  would 
be  in  a  direction  opposite  to  that  in  which  the  mercury 
vapor  will  act  as  a  conductor.  Hence,  during  the  complete 
cycle  the  cathode  is  continuously  negative,  and  the  current 
at  this  point  is  unidirectional.  Each  anode  passes  current 
during  half  the  wave,  the  first  anode  during  the  first  half 
and  the  second  anode  during  the  second  half.  The  use 
of  the  entire  wave  is  shown  by  the  oscillograph  records  in 
Fig.  131.  The  upper  curve  shows  the  current  from  one 


Fig.  131. 

anode,  and  the  lower  curve  the  simultaneous  current  from 
the  other. 

If  it  were  possible  to  maintain  the  arc  on  a  single-phase 
rectifier  without  auxiliary  apparatus,  the  resulting  wave 
of  continuous  current  would  be  a  pulsating  wave  of  the 
same  characteristics  as  the  alternating-current  wave,  the 
negative  portions  of  the  alternating-current  wave  appearing 
reversed  in  reference  to  the  zero  line.  A  wave  shape  of 
this  form  reaching  a  zero  value  twice  in  every  cycle  cannot 
exist,  because  if  the  current  falls  to  zero  for  even  an  infin- 
itesimally  short  time,  the  cathode  will  cease  to  give  off 


MOTOR   GENERATORS,  ETC.  239 

ionized  vapor  and  the  arc  will  be  extinguished.  Suitable 
reactances  are  therefore  provided,  of  which  the  discharge 
voltage  is  sufficient  to  maintain  the  arc  during  the  period 
when  the  alternating  current  is  passing  through  zero. 
These  reactances  also  serve  to  smooth  out  the  current 
pulsations  in  the  continuous-current  side  so  that  the 
current  at  the  cathode  is  not  only  unidirectional  but  one 
with  pulsations  of  commercially  negligible  amplitude. 


Fig,  132, 

The  resulting  direct-current  wave  formed  is  shown  at 
A  A  in  Fig.  132  with  reference  to  its  zero  line  XX.  Curve 
A  A  is  obtained  by  superimposing  the  two  curves  shown 
in  Fig.  131.  In  Fig.  132  the  wave  of  simultaneous 
impressed  alternating  E.M.F.  is  shown  at  the  bottom 
with  reference  to  its  zero  line  YY. 

The  action  of  the  reactances  is  seen  from  Fig.  133,  the 
upper  curve  showing  the  charge  and  discharge  voltage  of 
the  reactance  due  to  the  impressed  alternating  E.M.F. 


240     POLYPHASE  APPARATUS  AND  SYSTEMS. 

shown  by  the  lower  curve.  In  the  upper  curve  the  dis- 
charge voltage,  Which  is  that  value  below  the  zero  line 
XX,  is  seen  to  occupy  considerably  more  than  half  the 
cycle.  It  is  this  overlap  which  eliminates  the  zero  points 
mentioned  and  prevents  the  arc  from  being  extinguished. 

Referring  to  the  diagram  of  connections  (Fig.  134)  the 
cathode  B  forms  one  terminal  of  the  continuous-current 
circuit.  The  other  terminal  is  at  the  junction  D,  between 


Fig.  133, 

the  two  reactive  coils  E  and  F.  The  load  J  is  indicated 
as  a  storage  battery  which  is  being  charged.  Assuming 
an  instant  when  the  terminal  H  of  the  alternating-current 
supply  is  positive,  the  anode  A  is  positive,  and  the  current- 
carrying  arc  flows  from  A  to  B.  Following  the  direction 
of  the  arrows,  the  current  passes  through  the  external 
load  /,  through  the  reactance  E  and  back  to  the  negative 
alternating-current  terminal  G.  A  little  later  when  the 
E.M.F.  impressed  on  A  falls  below  the  value  sufficient  to 
maintain  the  arc  against  the  counter  E.M.F.  of  the  arc 


MOTOR   GENERATORS,  ETC. 


241 


and  the  load,  the  reactance  E,  which  has  heretofore  been 
charging,  now  discharges,  the  discharge  current  being  in 
the  same  direction  as  the  previous  charging  current. 
The  discharge  voltage  of  E  serves  to  sustain  the  arc  in 
the  rectifier  until  the 

E.M.F.  of  the   alter-  I 

i  ' v/WWVWWW ' 

natmg   supply  passes  ^Gnsform&T 

through  zero,  reverses         ^  j VWVWW \ 

and  builds  up  again 
to  a  value  that  makes 
A'  sufficiently  positive 
to  start  an  arc  be-  ®  J 
tween4'and£.  The 
discharge  circuit  of 
the  reactance  E  is 
now  through  the  arc 
A'B,  and  the  current 
flowing  between  A'  ^ 
and  B  in  the  rectifier 
is  due  in  part  to  that 
supplied  by  the  reac- 
tance E  and  in  part 
to  that  emanating 
from  the  terminal  G, 
which  has  now  be- 
come positive.  The  new  circuit  from  the  transformer  is 
indicated  by  the  arrows  inclosed  in  circles. 

The  initial  ionization  of  the  mercury  vapor  is  accom- 
plished by  the  starting  anode  C,  which  is  brought  into 
contact  with  the  cathode  by  tilting  the  tube  and  allowing 
the  mercury  to  bridge  across  the  terminals  B  and  C.  The 
breaking  of  this  mercury  bridge  when  the  tube  is  restored 


Fig.  134, 


242     POLYPHASE  APPARATUS  AND  SYSTEMS. 

to  a  vertical  position  starts  a  small  initial  arc  which  excites 
the  cathode,  and  this  causes  the  liberation  of  sufficient 
ionized  vapor  to  enable  the  main  anodes  to  become  active. 
A  small  resistance,  R,  connected  in  series  with  the  starting 
anode,  limits  the  flow  of  current  during  starting. 

The  action  of  the  rectifier  tube  is  practically  independent 
of  frequency.  The  reactances  used  with  it  should,  for 
best  results,  be  designed  for  a  frequency  near  that  on 
which  the  device  is  to  be  used.  Mercury  rectifiers  are 
adapted  for  use  on  circuits  of  a  very  wide  range  of  voltage. 
While  most  commonly  used  on  constant  potential  circuits 
of  one  or  two  hundred  volts,  they  are  equally  adapted  for 
higher  potentials,  and  in  special  designs  have  been  success- 
fully employed  to  deliver  continuous  current  at  a  pressure 
as  high  as  6000  volts  in  connection  with  constant-current 
series  arc-lighting  systems.  The  ratio  of  the  continuous 
to  the  alternating-current  voltage  is  slightly  less  than  0.5 
and  is  practically  constant  at  all  loads  and  voltages,  the 
ratio  decreasing  somewhat  as  the  load  increases,  due  to 
the  voltage  drop  in  the  windings  of  the  reactance. 

A  distinctive  feature  of  the  rectifier  is  the  practically 
constant  drop  of  approximately  14  volts  in  the  arc  itself. 
This  drop  is  not  in  the  nature  of  a  loss  of  power  in  resist- 
ance, but  is  a  counter  E.M.F.  which  does  not  change  with 
load,  frequency  or  delivered  voltage. 

Neglecting  losses  in  the  reactance,  it  is  therefore  seen 
that  the  efficiency  of  the  rectifier  will  be  higher  on  high- 
voltage  than  on  low-voltage  circuits,  for  the  percentage 
of  power  used  in  overcoming  the  counter  E.M.F.,  which 
has  always  the  constant  value  of  14  volts,  is  small  when 
the  delivered  voltage  is  high.  For  example,  the  efficiency, 
including  reactance  losses,  of  a  rectifier  delivering  a  con- 


MOTOR   GENERATORS,  ETC.  243 

tinuous  current  of  30  amperes  at  80  volts  will  be  above 
75  per  cent  at  all  loads  from  quarter  load  to  full  load. 
With  the  same  output  at  a  potential  of  no  volts  the  effi- 
ciency will  be  increased  to  80  per  cent.  These  efficien- 
cies are  very  striking  compared  with  those  obtainable 
from  a  motor-generator  set  of  equivalent  output,  from 
which  in  so  small  a  unit  it  would  be  difficult  to  obtain 
more  than  70  per  cent  even  at  full  load,  while  the  quarter- 
load  efficiency  would  be  hardly  better  than  40  per  cent. 

The  power  factor  averages  about  90  per  cent  over  con- 
siderable ranges  of  load.  This  value  is  also  materially 
higher  than  could  readily  be  attained  by  an  induction 
motor  generator  set  of  the  same  capacity. 

The  ampere  capacity  for  which  mercury  rectifiers  have 
thus  far  been  successfully  built  appears  limited  to  about 
40  amperes.  Where  the  conditions  require  a  larger  output 
than  can  be  supplied  from  a  single  apparatus,  parallel 
operation  is  satisfactorily  effected. 


244 


POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER  X. 
SWITCHBOARDS   AND   STATION    EQUIPMENT. 

Control  of  Alternating-Current  Apparatus.  —  The  rapid 
introduction  of  generators  of  extremely  large  output,  and 
the  use  of  currents  of  increasingly  high  pressures,  have 
demanded  a  corresponding  advance  in  the  development  of 
all  details  of  alternating-current  switchboard  equipment. 
Under  modern  conditions  large  districts  are  often  dependent 
upon  a  single  generating  station  for  the  supply  of  current 
for  lighting  or  for  power.  In  such  cases  even  a  temporary 
interruption  of  the  supply  is  most  serious,  and  elaborate 
precautions  are  taken  to  insure  continuity  of  the  service. 

The  successful  operation  of  modern  alternating-current 
distribution  systems  is  due  primarily  to  the  perfection  of 
the  oil  switch  and  circuit  breaker,  which  will  be  described 
below.  Protection  against  short  circuits  within  the  station 
is  secured  by  a  liberal  spacing  and  careful  insulation 
of  the  conductors.  In  large  plants,  especially  those  work- 
ing at  high  voltage,  all  conductors  of  opposite  polarity  are 
separated  by  masonry  barriers,  or  located  in  fire-proof 
compartments.  This  construction  is  used  especially  for 
the  bus  bars,  but  is  applied  also  to  the  machine,  trans- 
former and  feeder  circuits.  In  this  way  the  effects  of  a 
short  circuit  are  localized  as  far  as  possible  so  that  the 
operation  of  other  circuits  shall  remain  unaffected.  All 
these  provisions  for  the  proper  running  and  spacing  of  the 


SWITCHBOARDS   AND    STATION    EQUIPMENT.       245 

wiring,  and  for  the  convenient  location  of  the  various 
devices  which  constitute  the  modern  alternating-current 
switchboard  installation,  require  that  the  station  building 
shall  be  designed  with  a  full  knowledge  of  the  switching 
equipment  which  it  is  to  contain. 

Switches  and  Circuit  Breakers.  —  Before  the  develop- 
ment of  the  present  effective  devices  the  switching  of 
alternating-current  circuits  was  done  with  knife-blade 
switches  similar  to  those  familiar  in  continuous-current 
work,  the  principal  difference  lying  in  the  increased  length 
of  blade  so  as  to  give  a  greater  breaking  distance  suited  to 
the  higher  voltages.  Switches  of  this  type  for  use, on  cir- 
cuits as  high  as  25,000  volts  have  been  constructed ;with 
switch  blades  five  to  six  feet  long.  These  switches  require 
a  great  amount  of  room  on  account  of  the  sweep  of  the 
long  arms.  When  opened  under  load  vicious  arcing 
frequently  follows,  and  the  arc  may  hold  persistently,  the 
heated  vapors  serving  to  maintain  a  path  of  conductivity 
through  the  air.  Thus  under  severe  conditions  a  switch 
of  this  type,  even  though  of  exaggerated  dimensions,  may 
be  unable  to  open  the  circuit.  A  further  and  most 
important  disadvantage  is  that  even  though  such  an  arc 
may  ultimately  extinguish  itself,  a  phenomenon  of  rise  of 
potential  may  be  produced  when  interruption  of  the  cir- 
cuit takes  place.  This  is  often  sufficient  to  break  down  the 
insulation  of  such  apparatus  as  is  connected  to  the  circuit. 
It  was  soon  recognized  that  to  meet  the  new  conditions 
some  other  procedure  was  necessary  than  merely  to  enlarge 
the  dimensions,  without  modifying  the  type,  of  a  switch 
that  was  satisfactory  under  earlier  and  less  difficult  require- 
ments. Among  the  various  types  devised  none  were  fo.  nd 
to  meet  adequately  the  increasingly  severe  conditions  until 


246 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


the  advent  of  the  oil  switch.  For  practically  all  alternating- 
current  work  this  type  is  now  used  to  the  exclusion  of  all 
others. 


Fig.  135. 

The  switch  in  its  simplest  form  consists  of  one  or  more 
sets  of  contacts  by  which  the  circuit  is  made  or  broken 
beneath  the  surface  of  oil  contained  in  a  surrounding  vessel. 

A  typical  three-pole  oil-break  switch  with  oil  tank  remove 
is  shown  in  Fig.  135.  Connection  between  the  opposite 


SWITCHBOARDS    AND   STATION    EQUIPMENT.       247 


clips  of  each  pole  is  made  by  the  bridging  pieces  seen  at 
the  bottom.  These  are  connected  to  wooden  rods  which 
are  raised  and  lowered  simultaneously  by  a  system  of 
toggles  and  levers  connected  to  the  operating  handle  on 
the  front  of  the  board.  Where  the  potential  exceeds  about 
2,000  volts  these  switches,  instead  of  being  mounted  on  the 
back  of  the  panel,  are  preferably  mounted  on  a  separate 
framework  some  distance  away,  so  as  to  avoid  crowding 
the  wiring.  When  mounted  in  this  way,  connection  between 
the  switch  and 
the  operating 
handle  is  effected 
by  a  system  of 
rods  which  are 
usually  located 
below  the  floor 
line.  Or  the 
switch  may  be 
actuated  electri- 
cally by  means 
of  a  solenoid,  the 
circuit  of  which 

runs  to  a  small  controlling  switch  on  the  front  of  the 
panel.  This  type  is  illustrated  in  Fig.  136.  In  this 
photograph  separate  compartments  are  seen  to  be  pro- 
vided for  each  pole.  In  this  way  more  perfect  insulation 
is  secured  and  the  rupturing  capacity  increased. 

For  stations  of  the  largest  capacities  the  oil  switch  is 
constructed  so  that  each  break  of  each  phase  is  effected 
in  a  separate  oil  vessel.  A  two-phase  switch  of  this  form 
consists  of  fo.ur  single-phase  switches,  each  of  which  con- 
sists of  two  separate  contacts  each  inclosed  in  its  own  oil 


Fig.  136. 


248 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


vessel.  A  three-phase  switch  consists  of  three  such  pairs 
of  contacts.  All  the  contacts  of  all  the  phases  are  arranged 
to  be  closed  or  opened  simultaneously.  The  several 


Fig.  137. 

single-phase  elements  are  separated  by  masonry  barriers 
and  surmounted  by  the  operating  mechanism. 

The    general    arrangement    of    an    electrically  operated 
switch  of  this  form  assembled  in  its  compartments  is  shown 


SWITCHBOARDS    AND    STATION    EQUIPMENT.       249 


JQL 


in  Fig.  137.  The  internal  connections  of  a  single  element 
are  shown  in  outline  in  Fig.  138.  Each  single-phase 
element  consists  of  two  metal  cylinders  which  contain  the 
oil  and  the  contacts.  The  incoming  lead  is  attached  to 
one  cylinder,  and  the  outgoing  lead  of  the  same  phase  is 
attached  to  the  other. 
Two  copper  rods, 
joined  at  the  top  by  a 
metallic  cross-head, 
slide  through  an  insu- 
lating sleeve  and  make 
contact  at  the  bottom 
of  the  cylinders. 

The  switch  illus- 
trated in  Fig.  137  is 
electrically  operated  by 
a  small  motor  which 
drives  a  worm  gear. 
This  transmits  motion 
to  a  rocker  shaft 
through  a  friction 
clutch.  Motion  is  com- 
municated to  the  con- 
tact-making parts  by 
means  of  wooden  insu- 
lating rods  actuated  Fig.  138. 
from  the  three  arms 

which  are  keyed  to  the  rocker  shaft.  At  both  ends  of  the 
stroke  a  spring  is  compressed  by  the  action  of  the  motor 
and  held  in  such  a  way  that  if  current  is  applied  to 
the  motor  a  few  revolutions  of  its  armature,  requiring  a 
fraction  of  a  second,  will  release  the  spring  and  throw  the 


250     POLYPHASE  APPARATUS  AND  SYSTEMS. 

switch.  The  motor  continues,  to  revolve  until  it  has  com- 
pressed the  other  spring,  and  is  then  disconnected  by  means 
of  an  auxiliary  contact  on  the  operating  mechanism.  A 
second  application  of  current  to  the  motor  will  now  release 
this  spring  and  throw  the  switch  back  to  the  original  posi- 
tion, after  which  the  first  spring  is  again  compressed  and 
the  motor  disconnected  from  circuit  in  the  manner  described. 
The  circuit  to  the  motor  is  led  through  a  small  controlling 
switch  placed  on  the  front  of  the  switchboard  panel.  Suit- 
able pilot  lamps  are  provided,  the  respective  circuits  of 
which  pass  through  contacts  on  the  operating  mechanism. 
These  lamps  become  alight,  one  for  the  closed  and  one  for 
the  open  position,  and  inform  the  attendant  of  the  proper 
functioning  of  the  switch,  which  may  not  be  within  his 
view. 

The  switch  shown  is  designed  for  circuits  up  to  15,000 
volts.  It  occupies  a  floor  space  2  feet  7  inches  by  4  feet  6 
inches,  and  is  8  feet  high  over  the  top  of  the  operating 
mechanism.  The  movement  of  the  contact  rods  is  20 
inches.  When  built  for  60,000  volts  the  floor  space  is 
5  feet  2  inches  by  n  feet,  the  overall  height  is  u  feet  9 
inches,  and  the  rods  have  a  travel  of  34  inches. 

Switches  of  this  general  form  are  also  made  electrically 
operated  by  means  of  solenoids,  or  they  may  be  operated 
by  compressed  air. 

The  superiority  of  the  oil  switch  over  all  other  types 
thus  far  introduced,  lies  in  two  features  which  are  of  nearly 
equal  importance.  These  are  its  ability  to  interrupt  the 
circuit  with  great  certainty,  and  the  fact  that  practically  no 
rise  of  potential  ensues  when  the  circuit  is  broken.  The 
effective  action  of  the  switch  in  interrupting  the  circuit 
is  due  to  the  "cooling  action  which  the  oil  has  on  the  arc, 


SWITCHBOARDS  AND  STATION  EQUIPMENT. 

and  which  extinguishes  it  promptly  by  lowering  the  tem- 
perature below  the  value  at  which  the  arc  can  hold.  The 
absence  of  any  appreciable  rise  of  voltage  when  the  circuit 
is  broken  follows  from  the  fact  that  in  this  type  of  switch 
the  arc  is  broken  at  or  near  the  zero  point  of  the  wave. 

Whenever  an  alternating-current  circuit  is  interrupted 
there  is  a  definite  amount  of  electromagnetic  and  electro- 
static energy  stored  in  the  line  which  must  be  spent  in 
some  way.  The  magnitude  of  this  energy  depends  on  the 
inductance  and  capacity  of  the  circuit  and  on  the  instan- 
taneous value  which  the  current  has  at  the  moment  of 
interruption.  The  circuit  being  interrupted,  the  only 
path  that  can  be  taken  by  the  current  which  was  previously 
flowing  through  the  switch  is  now  formed  by  the  line 
capacity,  and  the  line  therefore  becomes  charged  to  a 
certain  potential  depending  on  the  line  constants,  this 
potential  being  proportionate  to  the  value  of  the  current 
at  the  moment  of  interruption.  In  the  air  switch  interrup- 
tion is  liable  to  take  place  at  the  maximum  of  the  current 
wave,  and  a  very  high  voltage  may  thus  result.  If,  on  the 
other  hand,  the  current  is  interrupted  when  at  zero  value, 
no  rise  of  potential  will  occur.  This  is  what  takes  place  in 
the  oil  switch  and  constitutes  a  feature  of  the  highest  value. 
This,  together  with  the  safety  and  general  effectiveness  of 
the  device,  marks  the  oil  switch  as  one  of  the  most  impor- 
tant developments  of  recent  years. 

Oil  for  use  in  these  switches  should  be  carefully  selected. 
The  requirements  are  in  general  the  same  as  should  be 
demanded  for  oil  used  in  transformers  of  equivalent  poten- 
tial. A  renewal  of  the  oil  is  necessary  from  time  to  time, 
since  its  properties  suffer  deterioration  through  the  gradual 
carbonizing  effect  produced  by  the  arcing  between  the 


252     POLYPHASE  APPARATUS  AND  SYSTEMS. 

contacts.  No  general  rule  can  be  given  covering  the  inter- 
val between  renewals,  as  this  will  depend  on  the  number 
of  times  the  switch  is  operated  and  on  the  severity  of  the 
load  which  it  interrupts.  In  general,  new  oil  is  seldom 
required  oftener  than  once  in  three  months,  and  it  is  fre- 
quently found  that  even  after  a  year's  service  the  oil  is  still 
sufficiently  good. 

The  various  types  of  switches  described  above  may  be 
equipped  with  a  tripping  device  to  cause  them  to  open 
automatically  under  given  conditions.  Thus  the  switch 
may  be  made  to  open  when  the  load  increases  to  a  prede- 
termined value,  its  action  in  this  respect  then  corresponding 
to  that  of  the  familiar  direct-current  circuit  breaker.  This 
overload  device  may  be  of  a  type  that  will  cause  the  switch 
to  trip  immediately  in  case  a  given  overload  is  reached,  or 
it  may  be  so  arranged  that  a  certain  time  will  elapse  before 
the  switch  will  function.  Thus  the  device  may  be  so 
adjusted  that  the  switch  will  not  open  unless  the  overload 
persists  for,  say,  five  seconds.  This  feature  is  useful  in 
.preventing  too  frequent  tripping  of  the  switches,  such  as 
during  overloads  of  very  brief  duration  which  would  not 
harm  the  apparatus  connected  to  the  circuit.  It  also 
tends  to  make  it  easier  for  the  switch  to  break  the  current 
in  the  event  of  a  severe  short  circuit,  because  the  switch 
does  not  open  till  after  the  first  heavy  rush  of  current  has 
passed.  When  constructed  in  this  form  the  overload  device 
is  said  to  have  the  " definite  time"  feature.  Another  form 
is  also  used  having  what  is  called  the  "inverse  time" 
feature.  This  takes  its  name  from  the  fact  that  the  time 
elapsing  between  the  application  of  the  overload  and  the 
tripping  of  the  switch  is  inversely  proportional,  or  nearly 
so,  to  the  amount  of  overload.  This  type  is  used  where 


SWITCHBOARDS   AND    STATION    EQUIPMENT.       253 

it  is  desired  to  have  the  switch  open  immediately  in  case 
of  very  heavy  overload,  while  responding  to  the  action  of 
moderate  overload  only  in  case  the  duration  of  the  excess 
current  is  considerable.  If  with  either  of  these  two  types 
the  overload  is  removed  before  the  expiry  of  the  time  limit 
for  which  the  device  is  set  the  switch  will  remain  closed. 

Fuses.  —  The  use  of  the  ordinary  open  fuse  on  high- 
potential  circuits  is  in  general  subject  to  the  same  limita- 
tions and  disadvantages  as  the  air-break  switch  in  respect 
to  fire  hazard,  unreliability  and  dangerous  rise  of  potential. 
Automatic  oil  switches  have  therefore  been  preferably 
employed  for  all  important  work.  In  new  designs  lately 
developed  the  fuse  blows  in  a  confined  space  of  such  a 
form  as  to  cause  the  arc  to  be  extinguished  at  the  zero 
point  of  the  wave,  as  in  the  oil  switch.  Fuses  of  satisfac- 
tory type  should  find  a  wide  application  in  situations  where 
moderate  powers  are  dealt  with  under  conditions  which 
from  the  commercial  standpoint  would  not  permit  the  use 
of  the  much  more  costly  oil  switch. 

Switchboards.  —  The  modern  alternating-current  switch- 
board, especially  for  large  powers  and  high  voltages,  is 
really  an  installation  in  itself,  consisting  of  many  separately 
located  parts.  The  type  and  size  of  the  oil  switches,  the 
space  demanded  by  the  bus-bar  structure,  and  the  necessary 
provision  of  liberal  room  for  all  high-tension  parts,  not  to 
mention  the  space  taken  up  by  the  switchboard  panels 
themselves,  require  sometimes  not  less  than  one-quarter 
of  the  cubic  contents  of  the  station  building  for  the  accom- 
modation of  the  switchboard  equipment.  Only  a  limited 
portion  of  the  equipment  can  be  mounted  on  the  panels; 
the  remainder  must  be  disposed  of  elsewhere  (see  Figs.  147 
and  148,  which  will  be  referred  to  later). 


254     POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  panels  may  be  either  of  marble  or  of  slate.  At 
2,000  volts  and  less,  live  parts  can  safely  be  mounted  on 
the  panels  if  they  are  of  marble.  If  of  slate,  they  should 
carry  no  live  parts  at  higher  than  600  volts,  the  occasional 
metallic  veins  in  this  material  rendering  it  inferior  as  an 
insulator.  It  is  safer  at  even  2,000  volts,  and  almost 
imperative  at  higher  voltages,  so  to  arrange  the  circuits 
that  the  panels  shall  carry  no  live  parts  that  are  electrically 
in  connection  with  the  main  circuits.  This  is  accom- 
plished by  locating  the  switches  and  bus  bars  at  a  distance, 
and  by  connecting  all  measuring  instruments  on  derived 
circuits  fed  from  special  instrument  transformers.  These 
are  well-insulated  transformers  in  which  the  primary  is 
connected  to  the  high-voltage  circuit  that  is  to  be  metered, 
the  secondary  leads  going  to  the  switchboard  instruments. 
The  frames  of  these  transformers  are  grounded,  as  is  also 
one  side,  or  the  neutral,  of  the  secondary  winding.  Allow- 
ance for  the  ratio  of  transformation  is  made  in  the  cali- 
bration of  the  instruments  so  that  these  may  read  directly 
in  the  units  actually  dealt  with.  By  this  means  it  is  possible 
to  keep  within,  say,  125  volts  the  potential  of  all  wiring 
which  must  be  carried  to  the  panels;  and  since  the  manipu- 
lation of  the  main  switches  is  effected  either  by  means  of 
an  insulated  system  of  levers,  or  electrically  by  means  of 
currents  at  low  pressure,  all  parts  of  the  board  with  which 
the  operator  can  come  in  contact  are  made  perfectly  safe. 
By  this  means  also,  slate  panels  become  as  satisfactory  as 
marble,  and  have  the  advantage  of  more  uniform  appear- 
ance owing  to  their  neutral  tint,  together  with  a  slightly 
lower  cost. 

The  panels  are  made  of  uniform  height,  and  as  far  as 
possible  of  the  same  width,  in  order  to  present  a  symmetrical 


SWITCHBOARDS    AND    STATION    EQUIPMENT.       255 

appearance.  They  are  arranged  side  by  side  and  bolted 
to  an  iron  framework.  The  measuring  instruments  are 
fastened  to  the  front  of  the  panel,  the  electrical  connec- 
tions for  them  being  made  behind  the  panel.  On  the 
front  of  the  panel  are  also  mounted  the  various  switch 
handles  and  rheostat  hand  wheels  within  easy  reach  of  the 


Fig.  139. 

operator.  For  large  stations  employing  many  electrically 
operated  switches  a  compact  arrangement  of  the  operating 
board  is  often  secured  by  grouping  the  various  controlling 
switches  on  a  sloping  table  or  "  bench-board"  in  front  of 
the  instrument  panels.  Such  an  arrangement  is  provided 
in  the  switchboard  illustrated  in  Fig.  139.  This  switch- 


256 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


board   controls   a   large   hydro-electric   station   having   an 
output  of  nearly  50,000  horse  power  at  60,000  volts. 


rt//< 

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ftrneter 

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Qoerat/npffuses 

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fitse- 

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f/e/ct  /f/?e< 

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Fig.  140.     Exciter  Panel. 

Typical  Connection  Diagrams.  —  The  switchboard  equip- 
ment   in     the    average    generating    station    will    include 


SWITCHBOARDS    AND    STATION    EQUIPMENT.       257 

panels  for  the  control  of  the  generators,  exciters,  and 
outgoing  lines,  each  machine  and  feeder  having  its  own 
panel. 

There  are  usually  two  exciter  panels,  as  the  exciters  are 
desirably  provided  in  duplicate,  each  of  sufficient  capacity 
to  excite  all  the  generators.  The  equipment  of  the  exciter 
panels  will  include  the  necessary  switches,  field  rheostat, 
and  an  ammeter.  A  single  voltmeter  will  suffice  for  two 
or  more  exciters,  the  voltmeter  being  connected  to  any  one 
at  will  by  means  of  a  small  plug  switch.  The  exciter  panels 
may  also  be  equipped  with  switches  for  the  station  lighting 
circuits  where  these  are  fed  from  the  exciter  generator. 
The  general  connections  of  the  exciter  panel  are  shown  in 
Fig.  140. 

On  each  generator  panel,  Fig.  141,  the  instrument  equip- 
ment will  include  an  ammeter,  voltmeter,  wattmeter,  and 
the  necessary  synchronizing  appliances.  A  power-factor 
meter  is  sometimes  supplied,  or  the  wattmeter  may  be 
made  to  indicate  the  wattless  component  of  the  output  by 
means  of  a  change  of  connections,  thus  serving  the  same 
purpose.  Where  an  unbalanced  load  is  anticipated,  an 
ammeter  is  provided  in  each  phase.  In  the  diagram 
illustrated  the  generator  is  seen  to  be  operated  in  conjunc- 
tion with  step-up  transformers.  These  are  tied  directly 
to  the  generator  terminals,  and  the  switching  is  done  on  the 
high-tension  side.  With  this  arrangement  each  generator 
with  its  transformers  is  treated  as  a  single  high-tension  unit. 
Where  the  transformer  banks  can  be  made  to  correspond 
in  capacity  and  number  with  the  generators,  this  method  of 
control  avoids  the  necessity  for  any  low-tension  bus  bars, 
instruments,  or  switching  equipment.  The  right-hand 
diagram  of  the  figure  applies  where  the  main  switch  is 


258 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


manually  operated   (as  in  Fig.   135).     The  left-hand  dia- 
gram shows  the  arrangement  used  when  the  main  switch 

With  electrically  operated  switch  With  hand  operated  switch 


Wattmeter^. 

Synchro/v'z/n&P/ugy 
rt/ny/ 

s/ 


Ajtentia/ . 
1tf reformers 

Fuse 


|  Connections  for  the  Cn$in§  > 
Governor  Con  trot  Motor 
anof  Switch  when  Svpp/iecf 

Current  Generator, 


Fig.  141.    Generator  with  Step-up  Transformers. 


is  electrically  operated,  including  the  control  switch  and 
the  connections  of  the  red  and  green  pilot  lamps.     Over- 


SWITCHBOARDS    AND    STATION    EQUIPMENT.       259 

loads  on  the  system  are  relieved  by  the  tripping  of  auto- 
matic switches  in  the  feeder  circuits.  The  generator 
switches  are  made  non-automatic,  since  otherwise  they  also 
would  trip  out  on  overload,  and  delay  in  restoring  power 
would  ensue  owing  to  the  time  required  to  resynchronize. 
In  the  diagram  referred  to  the  high-tension  transformer 
switch  may  be  regarded  as  the  generator  switch.  The 
sketch  shows  the  connections  of  the  current  and  potential 
transformers  for  the  instrument  circuits,  and  shows  that 
these  are  grounded,  as  previously  mentioned. 

Connections  for  a  generator  circuit  where  step-up  trans- 
formers are  not  used  are  given  in  Fig.  41,  on  page  61.  The 
same  description  applies  to  that  figure  as  to  the  one  just 
considered. 

For  the  outgoing  line,  or  high-tension  feeder,  the  panel 
connections  are  given  in  Fig.  142,  which,  like  the  preced- 
ing, covers  both  the  electrically  operated  and  the  hand- 
operated  type  of  switch.  The  switch  is  arranged  for  auto- 
matic trip,  generally  on  the  definite-time  principle.  The 
tripping  coils  are  usually  actuated  by  current  from  the 
exciters,  the  tripping  circuit  being  closed  by  a  relay  which 
in  turn  is  actuated  by  current  derived  from  a  current  trans- 
former in  the  main  circuit.  An  ammeter  is  provided  for 
each  phase,  to  give  evidence  of  any  unbalancing  or  of  open 
circuits  in  any  phase,  as  by  grounding  or  breaking  of  one 
of  the  line  wires. 

Lightning  arresters  and  choke  coils  are  connected  where 
the  line  leaves  the  building. 

Coming  now  to  the  receiving  or  substation  end  of  the 
transmission  line,  the  arrangement  for  the  control  of  the 
incoming  lines  is  seen  in  Fig.  143.  The  connections  and 
instruments  are  practically  the  same  as  for  the  outgoing 


260     POLYPHASE  APPARATUS  AND  SYSTEMS 

line,  except  that  only  one  ammeter  is  usually  needed.     The 
switch  is  of  the  automatic  type,  so  as  to  disconnect  the  sub- 

With  electrically  operated  switch  With  hand  operated  switch 


O//  5w/tch  operat/ng 
ffuses  on  Pane/ 


Lightning 
Arresters 

Fig.  142.     Outgoing  Line. 

station  from  the  source  of  power  in  the  event  of  severe 
overloads  or  short  circuits. 


SWITCHBOARDS   AND    STATION    EQUIPMENT.       261 

The  details  of  the  other  substation  panels  will  depend 
on  the  use  to  which  the  current  is  put,  i.e.,  whether  for 

With  hand  operated  switch    With  electrically  operated  switch 


Fig.  143.     Incoming  Line. 

lighting,    power,  or    railway  service.     Assuming    that    the 
substation    contains    rotary    converters    supplying    current 


262 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


to  a  railway  system,  the  machine  connections  are  as  shown 
in  Fig.   144.     In  this  figure  the  middle  diagram  applies 


With  hand 
operated  switch 


With  hand  With  electrically 

operated  switch  operated  switch 


Vo/tmeter  \/fmmeter\ 


rr/jt>  Co//         Switch 
Current  Transformer 


i    II I  Synchronizing  Buses 
~r7-.  Synchronizing 


Terminaf ff/oc/r 
on  Oi/Swtte* 


To  B tower  Motor 

Synchronizing  Connections  f Shown  DoUccOfor/totariea 
•SCartecf  fromO.C.  &>ct  or  by  /ncfuct/on  Motor 


ftotary  Converter 


Fig.  144.     Rotary  Converter,  Three-phase. 

where  the  step-down  transformers  are  connected  star  on 
the  high-tension  side,  and  the  other  two  diagrams  are  for 


SWITCHBOARDS    AND    STATION    EQUIPMENT.       263 


the  delta  high-tension  connection,   the    secondaries  being 
delta   in   all   three   cases.     The   converter   is   three-phase, 

With  hand  operated  switch          With  electrically  operated  switch 


inat 'B/oc/r  on 
O// Switch 


Synchronizing  Connections  fShown  Dotted) 
for  Notaries  Starting  /rom  D.  C.  Cnd 


To  3/ower  Motor 
\ffotary  Converter 


Fig.  145.     Rotary  Converter,  Six-phase. 

arranged  to  start  from  half  voltage  taps  on  the  transformers 
(see  page  218).     Dotted  lines  show  the  connections  which 


264     POLYPHASE  APPARATUS  AND  SYSTEMS. 

have  to  be  made  for  synchronizing  when  the  converter  is 
started  from  the  direct-current  end  or  by  a  starting  motor. 
Each  converter  is  fed  from  its  own  bank  of  transformers, 
and  all  switching  on  the  alternating-current  end,  except  for 
starting,  is  done  on  the  high-tension  side.  For  a  six-phase 
converter  the  connections  are  given  in  Fig.  145,  which 
differs  from  the  preceding  chiefly  in  the  starting  connections 
and  in  the  connections  of  the  transformer  secondaries, 
which  are  diametrical. 

On  the  continuous-current  side  of  the  converter  the 
arrangements  are  shown  in  Fig.  146.  An  ammeter  and 
recording  wattmeter  measure  the  output  of  each  machine. 
A  single  voltmeter  allows  the  voltage  of  any  machine  to  be 
read  by  means  of  a  plug  switch.  Small  switches  conven- 
iently located  control  the  lighting  circuits  of  the  substation. 
At  the  bottom  of  the  diagram  appear  the  connections  of  the 
shunt  field  with  the  switch  for  sectionalizing  the  winding 
when  starting. 

Location  of  Switchboard  Equipment.  —  Reference  has 
been  made  to  the  necessity  of  so  arranging  the  design  of  the 
power  house  that  sufficient  room  and  a  suitable  location  be 
provided  for  the  switchboard  equipment.  This  applies 
mainly  to  plants  of  large  capacity  or  those  working  at  high 
voltage,  since  in  the  case  of  small  or  low  voltage  plants  the 
switches  and  bus  bars  may  be  mounted  on  the  panels,  per- 
mitting an  arrangement  that  requires  but  moderate  space. 
As  an  example  of  the. former  type,  requiring  an  equipment 
of  the  most  powerful  switches  and  a  spacious  and  fire-proof 
arrangement  for  the  wiring,  a  typical  station  layout  is  shown 
in  Figs.  147  and  148.  These  two  cuts  show  the  transverse 
section  and  plan  of  a  water-power  station  now  under  con- 
struction, which  will  at  the  start  generate  6,000  H.P.  at 


SWITCHBOARDS    AND    STATION   EQUIPMENT.       265 

60,000  volts.     The  station  building  is  being  made  large 
enough    to    accommodate    additional    generators    in    the 


Fig.  146.     Kotary  Converter,  B.C.  End. 

future    sufficient    practically    to    double    the    present    ca< 
pacity. 


266 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


SWITCHBOARDS   AND    STATION    EQUIPMENT.       267 


268 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


The  operating  bench  board  from  which  the  station  is 
controlled  is  located  on  a  gallery  from  which  an  unobstructed 
view  of  the  machine  floor  is  obtained.  Back  of  the  operat- 
ing board  extend  in  two  rows  the  switches  for  generator, 
transformer,  and  line  circuits.  Beneath  the  gallery  are 
located  the  step-up  transformers,  each  in  a  fire-proof  com- 


•viH 

Reactive Co/7  \  \  I   !  Mr  lock 
with  Starting  \j    ' 

Ponef  V   .  .. 


^•-Ground  Connection 


Fig.  149. 


partment.  The  low-tension  bus  bars  extend  along  the 
gallery  behind  the  line  of  low-tension  switches.  The  high- 
tension  busses  are  located  on  the  main  floor  beneath  the 
high-tension  switches.  The  leads  from  generators  and 
exciters  and  from  the  low-tension  side  of  the  transformers 
consist  of  rubber-insulated,  lead-covered  cables  carried  in 
earthenware  ducts.  All  connections  of  the  high-tension 


SWITCHBOARDS    AND    STATION  EQUIPMENT.       269 

system  are  made  with  bare  copper  wire  supported  on 
insulators  of  the  same  type  as  are  used  on  the  transmission 
line.  The  location  of  the  switches  and  other  parts  aims  to 
provide  the  simplest  and  most  direct  layout  for  the  connec- 
tions between  switchboard  elements,  a  problem  which  in  the 


age  Detector 
Disconnecting  Switches 
L'/ghtning  Arresters 


T:±i 


Fig.  150. 


complexity  of  the  modern  plant  is  indeed  a  difficult  one. 
The  arrangement  of  apparatus  and  of  the  switchboard 
equipment  in  a  well-designed  rotary  converter  substation 
for  railway  service  is  illustrated  in  Figs.  149  and  150.  The 
high-tension  current  is  stepped  down  by  means  of  three- 
phase  air-blast  transformers,  the  location  of  which  together 
with  their  blower  sets  is  clearlv  shown. 


2  70 


POLYPHASE   APPARATUS    AND    SYSTEMS. 


Automatic  Regulation  of  Potential.  —  Probably  the  most 
satisfactory  device  thus  far  introduced  for  the  automatic 
regulation  of  generator  potential  is  found  in  the  Tirrill 
Regulator.  This  device  controls  the  voltage  of  the  gen- 
erator by  automatically  changing  the  voltage  of  the  exciter 


circuit  and  thus  altering  the  excitation  current  of  the  alter- 
nator. The  variation  of  the  exciter  voltage  is  effected  by 
a  set  of  contacts  on  the  regulator  which  intermittently  short 
circuit  the  rheostat  in  the  shunt  field  of  the  exciter.  Re- 
ferring to  the  diagram  of  connections,  Fig.  151,  C  is  an 


SWITCHBOARDS    AND    STATION    EQUIPMENT.        2/1 

alternating-current  magnet  having  a  potential  winding 
connected  to  the  bus  bars  through  a  potential  transformer. 
The  core  of  this  magnet  is  supported  partly  by  the  attrac- 
tion of  the  potential  winding  and  partly  by  the  counter- 
weight B  at  the  opposite  end  of  the  pivoted  lever.  With  an 
increase  of  load  on  the  alternator  the  voltage  tends  to  drop, 
and  this  lessens  the  current  flowing  through  the  potential 
winding  of  the  magnet.  The  core  will  therefore  fall  and 
bring  the  main  contacts  together.  An  auxiliary  circuit  is 
thus  established  through  the  differential  relay  D,  causing 
the  relay  contacts  to  close.  These  relay  contacts  are  con- 
nected across  the  exciter  rheostat,  which  is  thus  short- 
circuited.  The  exciter  voltage,  now  vigorously  augmented, 
brings  up  the  alternator  voltage  till  the  increase  of  current 
through  the  magnet  coil  raises  the  core  and  separates  the 
main  contacts  again.  This  acts  in  turn  to  separate  the 
relay  contacts  and  to  put  the  exciter  rheostat  again  in  cir- 
cuit, thereby  preventing  further  rise  of  voltage. 

In  the  action  of  the  device  the  contacts  are  continually 
opening  and  closing  with  great  rapidity,  with  the  result 
that  the  exciter  voltage  is  caused  to  assume  such  a  value  as 
will  give  the  necessary  excitation  to  the  alternators  for  any 
condition  of  load.  The  device  may  be  adjusted  to  give 
constant  bus-bar  voltage;  or  by  means  of  a  compensating 
winding,  the  effect  of  overcompounding  may  be  secured. 
The  appearance  of  the  apparatus  is  shown  in  Fig.  152,  the 
parts  being  lettered  to  correspond  with  the  diagram. 

Feeder  Regulators.  —  In  order  to  insure  correct  voltage 
at  a  number  of  different  points  fed  from  the  same  station, 
it  is  usually  necessary  to  provide  some  means  of  controlling 
individually  the  potential  of  the  separate  feeders.  This 
purpose  is  effected  by  devices  known  as  pressure  regulators, 


2/2 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


or  feeder  regulators.  They  are  made  in  several  types,  all 
being  virtually  one  or  another  form  of  variable  ratio  trans- 
former with  the  primary  connected  across,  and  the  second- 
ary in  series  with,  the  feeder  to  be  regulated.  The  product 


Fig.  152. 

of  the  volts  and  amperes  on  the  generator  or  bus-bar  side 
is  always  equal  to  the  product  of  the  volts  and  amperes  on 
the  feeder  side,  minus  the  small  loss  in  the  regulator  itself. 
This  is  shown,  neglecting  losses,  by  the  diagram  of  Fig. 
153,  drawn  for  a  single-phase  regulator  used  on  a  circuit 


SWITCHBOARDS    AND    STATION    EQUIPMENT.       273 

of  ioo  amperes  at  100  volts,  the  regulator  having  a  capacity 
to  raise  (or  lower)  the  voltage  of  the  circuit  by  10  per  cent. 
In  this  figure  the  values  are  taken  with  the  regulator  in  the 
maximum  boosting  position,  while  Fig.  154  shows  the  values 
corresponding  to  the  position  of  maximum  lowering  effect. 

In  one  form  of  regulator,  the  primary  and  secondary 
coils  are  wound  on  the  same  core,  as  in  a  transformer,  and 
a  number  of  taps  are  brought  out  from  the  secondary  wind- 
ing. These  are  connected  to  a  dial  switch,  and  the  voltage 
variation  is  made  by  moving 
the  switch  to  any  desired  rtovo/ts  too  Amps. 
contact.  The  connections 


To 
of  a  single-phase  regulator      Ge"*™*or 


£     ,  .  .  . 

of  this   type   are    given    in 
Fig.    155.       When    wound  Fig.  153. 

polyphase  a  similar  arrange- 

ment is    employed,  consisting  essentially  of  the  use  of  as 
many  single-phase  regulators  as  there  are  phases.     This 

form   of    regulator    may    be 
Govo/ts///.//Amp9.  f     referred    to    as    the     switch 
TO          ^P^      Tt  wil1  be  seen  that 
_  /*  there  is  a  definite  potential 

series  wincf/ny  interval   between    the    steps, 

Fig.  154.  which  depends  on  the  num- 

ber of  switch  contacts. 

In  another  form  of  regulator  constructed  both  for  single- 
phase  and  for  polyphase  circuits,  the  two  windings,  primary 
and  secondary,  are  each  placed  on  separate  circular  and 
concentric  slotted  cores  of  laminated  iron,  one  of  which, 
called  the  stator,  is  stationary,  and  the  other  of  which, 
termed  the  rotor,  is  arranged  so  that  it  may  be  angularly 
displaced  with  reference  to  the  stator.  This  form  of  regu- 


2/4 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


lator  is  known  as  the  induction  type,  and  has  the  important 
feature  that  it  gives  a  perfectly  smooth  curve  of  potential 

change,  as  contrasted 
with  the  step  by  step 
change  of  the  switch 
type,  due  to  the  fact 
that  the  potential  gener- 
ated changes  with  every 
alteration  in  the  relative 
positions  of  stator  and 
rotor.  This  is  well  illus- 
trated by  the  curves  in 

Fig.  156.     From  these  curves  it  will  also  be  seen  that  at 
no  load  the  boosting  and  lowering  effects  are  equal,  while  at 


2600 
ZSOO 

2300 
2200 
2100 

•^ 

•x, 

s^ 

X; 

\ 

X 

\»/V< 

?L 

OA 

D 

N^\ 

/ 

~UL 

Li 

OS 

D 

X^ 

V 

OL  T5  GENERA  TO  ft 

sX, 

<^ 

XX 

X 

X 

s^ 

<; 

^, 

O       SO      40       6O       BO  SO '00 

OfA/fMATL 

Fig.  156. 


full  load  the  lowering  is  always  somewhat  greater  than  the 
boosting  for  a  given  setting  of  the  regulator.  This  is  ex- 
plained by  the  fact  that  the  ohmic  and  reactive  drop  in 
the  windings  (which  tends  to  make  the  regulator  give  a 


SWITCHBOARDS    AND    STATION   EQUIPMENT.       2/5 


lower  voltage  when  loaded  than  when  on  open  circuit) 
is  additive  when  the  regulator  is  lowering,  and  is 
subtractive  when  the  regulator  is  boosting.  This  differ- 
ence, in  other  words,  is  the  analogue  of  regulation  in  a 
transformer.  In  the  usual  construction  the  secondary  or 
series  coil  is  wound  on  the  inside  circumference  of  the 
stationary  core,  and  the  primary  or  shunt  coil  is  wound 
in  the  slots  provided  on 
the  outside  circumference 
of  the  movable  core. 

In  the  single-phase  type 
the  connections  and  ar- 
rangement of  windings 
are  given  by  Fig.  157. 
The  primary,  or  rotor, 
contains  two  windings,  — 
the  active  or  shunt  wind- 
ing, connected  across  the 
line,  and  a  second  winding 
short-circuited  on  itself 
and  arranged  at  right 
angles  to  the  shunt  wind-  Fig.  157. 

ing,   the   purpose  of    the 

second   winding  being  to  decrease  the   reactance    of   the 
regulator. 

In  the  polyphase  induction  regulator  the  arrangement 
of  parts  and  type  of  winding  resemble  closely  the  construc- 
tion qf  an  induction  motor,  as  seen  for  example  in  iFig.  158, 
which  illustrates  the  stator  of  a  2 5 -kilowatt,  three-phase 
regulator.  Both  primary  and  secondary  have  a  definite 
polar  winding  embedded  in  slots  on  the  inner  and  outer 
surfaces  respectively  of  the  stator  and  rotor.  The  flux 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


which  generates  the  secondary  voltage  is  rotative,  as  in  an 
induction  motor.  It  has  a  practically  constant  value,  and 
hence  generates  in  the  secondary  a  voltage  which  is  prac- 
tically constant  and  which  is  independent  of  the  relative 
angular  displacement  of  the  rotor.  The  variation  of 
potential  resulting  from  the  action  of  the  regulator,  or,  in 
other  words,  its  ability  to  increase  or  decrease  the  voltage 


Fig.  158. 

of  a  circuit,  depends  on  the  fact  that  a  change  in  the  angular 
shift  of  the  rotor  produces  a  change  in  the  phase  displace- 
ment existing  between  the  constant  potential  delivered  to 
the  regulator  and  the  constant  potential  generated  in  its 
secondary,  the  magnitude  of  the  resultant  or  delivered 
potential  varying  with  the  magnitude  of  this  phase  dis- 
placement. 

This  may  be  graphically  illustrated,  as  in  Fig.  159.     Let 
EO  represent  the  normal  potential    or  bus-bar  voltage  of 


SWITCHBOARDS  AND  STATION  EQUIPMENT. 

one  phase  of  the  system,  and  let  the  radius  OB  represent 
the  constant  voltage  induced  in  the  secondary  of  the  regu- 
lator. With  the  primary  coil  of  phase  i,  for  example, 
directly  opposite  the  secondary  coil  of  the  same  phase,  the 
voltage  generated  will  be  OD,  opposed  to  the  bus-bar 
voltage,  and  the  regulator  will  lower  by  the  maximum 
amount,  making  the  resultant  or  feeder  voltage  equal  to 
ED.  As  the  primary  is  rotated  out  of  this  position,  say, 
about  40  degrees,  or  to  C,  the  resultant  of  EO  and  OC 
is  EC,  which  is  equal  to  EX.  Rotating  the  primary 
through  an  angle  of  nearly  90  degrees,  or  to  OB,  so  that 
EB  equals  EO,  the  regu- 
lator is  in  the  neutral  E  p  x  ^  o A 

position    and    neither         ^^          '    *     -''"  ' 

boosts  nor  lowers.  Com- 
pleting the  full  range  of 
1 80  electrical  degrees,  the 
secondary  voltage  OA  is 

directly  additive  to  the  bus-bar  voltage  EO,  and  the  re- 
sultant or  feeder  potential  is  EA,  the  regulator  now  being 
in  the  position  of  maximum  boost. 

Comparing  the  single-phase  and  polyphase  types  of 
induction  regulator  above  described,  the  important  differ- 
ence will  have  been  noted  that  in  the  former  the  generated 
secondary  voltage  is  in  phase  with  the  primary  E.M.F., 
and  is  variable  because  the  amount  of  flux  through  the 
secondary  changes  with  the  angular  position  of  the  primary, 
while  in  the  latter  the  flux  acting  through  the  secondary  and 
the  voltage  generated  in  the  secondary  are  constant,  the 
value  of  the  resultant  voltage  depending  on  an  actual 
phase  displacement  between  primary  and  secondary 

E.M.F:S. 


278     POLYPHASE  APPARATUS  AND  SYSTEMS. 

Regulators  of  the  induction  type  are  adapted  either  for 
hand  operation  or,  especially  in  the  larger  sizes,  for  elec- 
trical operation  by  means  of  a  small  motor,  the  motion  of 
the  actuating  shaft  being  communicated  to  the  rotor  through 
a  worm  wheel.  When  electrically  operated  they  may  be 
made  to  control  automatically  the  voltage  at  a  distant  point 
by  having  the  action  of  the  driving  motor  made  dependent 
on  a  relay  controlled  by  pressure  wires  from  the  end  of  the 

feeder,  or  by  equivalent  means. 
The  appearance  of  an  electrically 
operated  unit  is  shown  in  Fig.  160. 
Besides  their  use  for  the  con- 
trol of  feeder  voltage,  polyphase 
regulators  are  frequently  em- 
ployed in  conjunction  with  rotary 
converters  as  a  means  of  varying 
the  continuous-current  voltage. 
When  so  used  they  are  generally 
connected  in  the  low-tension  cir- 
cuit between  the  transformers 
and  the  converter  collector  rings 
Fig  160.  as  described  in  Chapter  VIII. 

The  induction  regulator  lends  itself  readily  to  the  con- 
struction of  very  large  units  where  the  conditions  demand 
a  considerable  range  of  voltage  on  heavily  loaded  feeders. 
Regulators  as  large  as  800  kilowatts  capacity  have  been 
successfully  constructed,  and  even  larger  outputs  are  feasible. 
A  regulator  of  the  size  mentioned  would  be  able  to  raise  or 
lower  by  5  per  cent  (equivalent  to  a  total  range  of  10  per 
cent)  the  voltage  of  a  circuit  carrying  a  load  of  16,000  kilo- 
watts, or  to  give  a  total  voltage  range  of  20  per  cent  on  a 
circuit  of  8,000  kilowatts. 


SWITCHBOARDS   AND    STATION    EQUIPMENT.        279 


The  approximate  weight  and  dimensions  of  standard 
three-phase  low-potential  induction  regulators  in  the 
smaller  sizes  ordinarily  used  is  shown  by  the  following 
table,  which  covers  regulators  boosting  or  lowering  by  10 
per  cent: 


Kilowatt  Capacity 
of  Regulator. 

Diameter  of 
Base. 

Height. 

Weight  Lbs. 

9-5 

26" 

48" 

1400 

19.  o 

26" 

54" 

1800 

38.0 

35" 

66" 

3400 

76.  o 

35" 

66" 

4200 

Power  Factor  and  Efficiency  of  Regulators.  —  The  power 

factor  of  the  induction  type  may  be  as  low;a^  85  per  cent, 
while  that  of  the  switch  type  is  considerabl)Hiigher.  Even 
this  apparently  low  power  factor  has  no  appreciable  effect 
on  the  power  factor  of  the  circuit  controlled,  owing  to  the 
fact  that  the  kilowatt  capacity  of  the  regulator  is  so  small 
a  proportion  of  the  energy  delivered  by  the  circuit  in  which 
it  is  connected. 

The  efficiency  in  sizes  from  5  to  25  kilowatts  in  the  poly- 
phase type  will  range  from  88  per  cent  to  92  per  cent  at 
full  load.  In  larger  sizes  the  efficiency  reaches  95  per  cent. 
Thus  in  small  sizes  the  losses  will  amount  to  from  8  to  12 
per  cent  of  the  capacity  of  the  regulator,  equivalent  to 
about  0.4  to  0.6  per  cent  of  the  capacity  of  the  circuit  con- 
trolled, assuming  the  amount  of  boost  or  reduction  to  be 
5  per  cent. 

Methods  of  Cooling.  —  Since  the  losses  in  a  polyphase 
induction  regulator  are  about  twice  as  great  as  in  a  trans- 
former of  the  same  kilowatt  output,  it  follows  that  artificial 
cooling  must  be  resorted  to  in  relatively  small  sizes.  Regu- 


28o     POLYPHASE  APPARATUS  AND  SYSTEMS. 

lators  of  the  oil  type  can  be  self-cooled  up  to  about  40 
kilowatts  capacity.  In  larger  sizes  they  are  equipped  with 
a  water-cooling  coil,  as  similarly  used  in  large  transformers, 
or  they  may  be  constructed  in  the  air-blast  type. 

Line-Drop  Compensator.  —  Whatever  method  is  adopted 
for  compensating  for  the  voltage  drop  in  feeders,  whether 
by  altering  the  bus-bar  voltage  or  by  the  use  of  individual 
feeder  regulators,  it  is  necessary  in  order  to  secure  good 
regulation  that  some  means  be  employed  that  will  at  all 
times  indicate  in  the  station  the  voltage  that  is  being  deliv- 
ered at  the  receiving  end  of  the  feeder.  While  this  can  be 
accomplished  by  pressure  wires  run  from  the  end  of  the 
feeder  back  to  the  station,  where  they  are  connected  to  a 
voltmeter,  tHs  method  is  cumbersome  and  expensive, 
especially  in  we  case  of  numerous  and  long  feeders.  The 
equivalent  result  is  secured  by  a  device  invented  by 
Mr.  R.  D.  Mershon,  which  may  be  termed  a  line-drop 
compensator,  the  principles  of  which  are  explained 
below. 

Suppose  that  at  the  station  there  were  artificially  pro- 
duced three  E.M.F.'s  proportional  to,  and  in  phase  with, 
respectively,  the  E.M.F.  impressed  upon  the  line  at  the 
station,  the  E.M.F.  consumed  by  the  reactance  of  the 
line,  and  the  E.M.F.  consumed  by  the  resistance.  If  it  be 
also  supposed  that  these  three  separate  E.M.F  Ss  are  com- 
bined in  the  same  way  as  are  their  counterparts  in  the  line, 
it  follows  that  their  resultant  will  be  equal  to  and  in  phase 
with  the  E.M.F.  at  the  far  end  of  the  line,  and  that  a  volt- 
meter actuated  by  this  resultant  E.M.F.  will  read  the  deliv- 
ered voltage.  The  device  in  question  effects  this  result  by 
providing  in  the  station  a  miniature  counterpart  of  the  line, 
having  the  same  relative  leactance  and  resistance,  and 


SWITCHBOARDS    AND    STATION    EQUIPMENT.       28 1 

through  this  artificial  circuit  the  voltmeter  is  connected  as 
shown  below. 

In  the  diagram,  Fig.  161,  the  potential  transformer  C 
gives  at  its  secondary  terminals  an  E,M.F.  proportional 
to  that  of  the  generator  G.  The  inductive  resistance,  a, 
and  the  ohmic  resistance,  b,  are  adjusted  with  respect  to 
the  current  from  the  series  transformer,  d,  so  that  with  a 


© 


•NVWW- 


rc 


V.M. 


Fig.  161. 

given  current  flowing  through  them,  the  reactive  and  ohmic 
drop  across  a  and  b  respectively,  have  the  same  value 
relative  to  the  E.M.F.  of  transformer  C  as  the  reactance 
and  resistance  E.M.F.'s  of  the  line  have  to  the  generator 
E.M.F.  Hence,  the  voltmeter  VM  reads,  the  voltage  of  C 
reduced  by  the  drop  across  a  and  b,  —  in  other  words,  it 
gives  the  same  reading  as  it  would  if  it  were  at  the  receiving 
end  of  the  circuit. 


282     POLYPHASE  APPARATUS  AND  SYSTEMS. 


CHAPTER    XI. 

LIGHTNING    PROTECTION    AND    LINE 
CONSTRUCTION. 

Lightning  Protection.  —  There  is  no  problem  with  which 
the  electrical  engineer  has  to  deal  that  presents  greater 
difficulties  in  the  way  of  a  positive  solution  than  that  of 
lightning  protection.  The  uncertainty  among  the  highest 
authorities  as  to  the  exact  nature  of  lightning  phenomena  is 
partly  accountable  for  this  state  of  affairs.  The  oscillatory 
character  of  the  direct  lightning  stroke  has  been  established 
beyond  a  doubt,  but  experience  with  lightning  effects  would 
indicate  that  the  frequency  of  the  oscillation  is  of  widely 
varying  characteristics.  For  this  reason,  no  one  single 
device  can  be  infallibly  depended  on  to  protect  electrical 
apparatus  from  all  kinds  of  lightning  phenomena.  In 
other  words,  there  is  not,  and  cannot  be,  a  universal  light- 
ning arrester. 

Under  the  term  lightning  protection  is  understood  to  be 
included  protection  against  high-potential  phenomena  of 
all  sorts,  whether  produced  by  lightning  or  other  atmos- 
pheric disturbances,  or  due  to  conditions  arising  in  the  cir- 
cuit itself. 

Excess  voltages  traceable  to  lightning  or  to  other  causes 
extraneous  to  the  circuit  may  be  divided  into  three  principal 
classes,  as  follows: 

First.  The  true  lightning  discharge,  as  when  the  trans- 
mission lines  are  in  the  direct  path  of  the  stroke. 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION-  283 

Second.  The  cumulative  discharge  due  to  a  gradual 
and  sometimes  enormous  rise  of  potential  from  a  changing 
electrostatic  condition  of  the  atmosphere.  This  class  of 
phenomenon  is  probably  of  the  same  nature  as  the  excess 
potentials  that  may  exist  where,  on  a  long  transmission,  one 
portion  of  the  line  is  subjected  to  different  atmospheric 
conditions  from  those  prevailing  in  another  part.  This 
condition  seems  to  be  frequent  where  there  is  considerable 
difference  of  elevation  in  different  portions  of  the  circuit,  as 
where  the  line  crosses  a  mountain  range. 

Third.  The  secondary  discharge  due  to  secondary 
currents  induced  in  the  line  by  a  parallel  lightning  stroke. 
In  this  case  the  line  plays  the  part  of  a  transformer  second- 
ary, regarding  the  path  of  the  lightning  stroke  as  the 
primary. 

Among  the  abnormal  potentials  arising  from  conditions 
within  the  circuit  are  those  due  to  switching,  grounds,  and 
short  circuits.  These  are  of  the  kind  referred  to  on  page 
251,  in  the  discussion  of  the  oil  switch,  and  are  frequently 
spoken  of  as  surges. 

The  high-voltage  disturbance  resulting  from  any  of  the 
above-mentioned  phenomena  is  alternating  in  character. 
The  frequency  of  the  alternations,  particularly  the  fre- 
quency of  lightning  discharges,  has  long  been  a  subject 
of  discussion  among  engineers,  and  while  it  is  generally 
agreed  that  the  surge  effects  due  to  switching,  short  circuits, 
and  grounds  are  usually  of  moderate  frequency,  say  about 
one  thousand  cycles  or  so,  close  observation  has  indicated 
that  in  some  cases  the  voltage  disturbance  is  one  having 
an  enormously  high  frequency.  To  be  effective  under  all 
conditions,  therefore,  lightning  arresters  should  be  able  to 
relieve  the  system  of  high-potential  stresses  at  any  frequency 


284 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


encountered  in  service.  They  should  also  be  able  to  inter- 
rupt promptly  and  with  certainty  the  flow  of  current  from 
the  line  which  tends  to  follow  every  static  discharge.  To 
fulfill  both  these  requirements  in  an  eminent  degree  has 
proved  a  difficult  problem,  which  is  still  engaging  the  most 
careful  study  and  experiment. 

Commercial    Types    of   Lightning    Arrester.  —  Although 
much  is  yet  to  be  learned  about  the  most  suitable  form  of 

lightning  protection  for 
alternating-current  cir- 
cuits, experience  has  nar- 
rowed down  the  many 
ancient  devices  to  one  or 
two  principal  types.  The 
simplest  of  these  is  the 
so-called  " goat- horn"  ar- 
rester shown  in  outline 
in  Fig.  162.  This  con- 
sists simply  of  two  wires 
bent  away  from  each 
other  like  a  pair  of  goat 
horns,  and  separated  at 
the  bottom  by  a  space 


Fig.  162. 


appropriate  to  the  voltage  at  which  the  arrester  is  in- 
tended to  discharge.  Connections  are  made  at  the  base 
of  each  horn  to  line  and  to  ground  respectively.  The 
current  from  the  line  which  follows  the  discharge  estab- 
lishes an  arc,  which  travels  upward  along  the  horns  under 
the  influence  of  the  heated  vapor  and  of  magnetic  repulsion, 
growing  longer  and  longer,  due  to  the  spread  of  the  horns, 
till  it  is  finally  extinguished.  A  series  resistance  is  usually 
inserted  to  limit  the  current  flow,  which  would  otherwise 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION    285 

rise  to  an  excessive  value,  being,  in  fact,  that  due  to  short 
circuit. 

This  type  of  arrester  is  cheaply  installed,  and  may  be  set 
up  out  of  doors  where  danger  from  fire  due  to  the  arc 
is  avoided.  When  used  without  series  resistance  it  has  the 
advantage  of  having  a  practically  constant  break-down 
point  on  all  frequencies ;  but  since  a  series  resistance  must 
usually  be  employed,  this  feature  loses  something  in  im- 
portance. A  series  resistance,  moreover,  when  used  with 
any  type  of  lightning  arrester,  is  now  considered  detrimental 
inasmuch  as  by  skin  effect  it  interposes  considerable  react- 
ance to  discharges  of  high  frequency,  and  correspondingly 
limits  the  range  of  the  arrester. 

The  other  chief  type  of  arrester,  and  the  one  which  is 
mainly  used  in  this  country,  consists  of  a  number  of  metal 
balls  or  cylinders  separated  by  short  air  gaps.  A  number 
of  these  cylinders  are  mounted  on  a  porcelain  base,  form- 
ing a  so-called  unit,  and  sufficient  units  are  connected  in 
series  so  that  the  total  gap  space,  i.e.,  the  aggregate  sum 
of  the  individual  gaps,  shall  be  appropriate  to  the  voltage 
at  which  the  arrester  is  intended  to  discharge.  A  unit  of 
this  type  is  illustrated  in  Fig.  163.  There  are  33  cylinders, 
each  i  inch  high  and  I  inch  diameter,  the  gap  space  between 
cylinders  being  -%%  inch.  No  series  resistance  is  used  with 
this  type  of  arrester,  but  it  is  customary  to  connect  a  shunt 
resistance  around  about  half  the  total  number  of  gaps. 
This  shunt  resistance  is  found  to  assist  materially  the 
interruption  of  the  resulting  current-flow  without  affecting 
detrimentally  the  ability  of  the  device  to  discharge  high- 
frequency  voltages.  The  arc- extinguishing  action  of  these 
arresters  is  dependent  mainly  upon  the  cooling  effect  of 
the  many  metal  cylinders,  the  large  aggregate  surface  of 


286     POLYPHASE  APPARATUS  AND  SYSTEMS. 

which  prevents  the  formation  of  any  heated  conducting 
vapor,  and  so  insures  that  the  arc  shall  be  promptly  extin- 
guished at  the  zero  point  of  the  wave  after  the  lapse  of  only 
two  or  three  cycles  at  most.  In  this  respect  the  action  of 
this  type  is  considered  superior  to  that  of  the  horn  arrester, 
where  it  frequently  happens  that  the  arc  is  not  extinguished 
till  after  several  seconds.  Considerable  importance  is  also 
attached  to  the  metal  of  which  the  cylinders  are  made, 
which  is  commonly  a  compound  of  bronze,  of  which  the 
vapor  is  understood  to  have  certain  non-conducting  prop- 
erties. 


Fig.  163. 

The  general  appearance  of  the  cylinder  type  of  arrester 
when  assembled  complete,  is  shown  in  Fig.  164,  which 
gives  the  connections  used  on  a  three-phase  circuit,  and 
which  also  shows  the  shunt  resistance  and  the  method  of 
connecting  it.  The  arrester  illustrated  is  suited  for  use  on 
a  circuit  having  a  working  potential  of  12,000  volts  between 
lines.  At  the  top  of  the  figure  is  seen  a  set  of  switches 
by  which  the  arrester  may  be  disconnected  from  the  line 
for  inspection. 

As  already  indicated,  the  number  of  units  which  should 
be  included  in  an  arrester  connected  to  any  system  is 
affected  by  the  length,  elevation,  and  insulation  of  the  line, 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION.  287 

as  well  as  by  the  load  conditions  and  other  factors,  so  that 
it  becomes  necessary  to  adjust  an  arrester  to  the  particular 
circuit  to  which  it  is  connected.  This  is  conveniently 
effected  by  means  of  adjustable  needle  gaps  (seen  just 
beneath  the  disconnecting  switches  in  the  figure)  which 


Fig.  164. 

can  be  set  for  any  desired  potential,  after  which  more  or 
fewer  of  the  cylinder  gaps  are  short  circuited  by  small  metal 
strips  till  the  discharge  will  by  preference  pass  through  the 
arrester  rather  than  across  the  needle  points. 

Installing  Lightning  Arresters.  —  The  principle  on  which 
a  lightning  arrester  is  selected  for  any  particular  voltage, 


288     POLYPHASE  APPARATUS  AND  SYSTEMS. 

is  that  it  must  be  the  weakest  point  in  the  line,  —  in  other 
words,  the  adjustment  must  be  such  that  any  excess  poten- 
tials will  be  discharged  through  the  arrester  rather  than  find 
a  path  to  ground  through  transformers  or  other  electrical 
apparatus  which  the  arrester  is  intended  to  protect.  It  is 
customary  to  install  arresters  at  each  end  of  each  line,  in 
the  generating  station  and  in  the  substation  respectively. 
In  the  case  of  long  transmissions  it  is  also  considered  advis- 
able to  install  one  or  more  sets  of  arresters  along  the  line, 
at  distances  of,  say,  ten  to  twenty  miles,  or  at  points  where 
excess  potentials  are  frequent.  In  the  case  of  local  distri- 
bution circuits  at  one  or  two  thousand  volts,  arresters  are 
installed  on  the  poles  at  intervals  of  about  a  thousand  feet, 
and  are  also  advisably  used  wherever  there  is  a  group  of 
transformers. 

In  the  installation  of  arresters,  whether  in  the  station 
or  on  the  line,  the  ground  connection  must  be  made  with 
the  utmost  thoroughness  and  care,  for  on  the  continuity 
and  effectiveness  of  the  ground  connection  the  operation 
of  the  arrester  absolutely  depends.  The  ground  is  pref- 
erably made  with  a  copper  sheet  about  yg-  inch  thick  and 
having  at  least  four  square  feet  of  surface,  buried  in 
powdered  coke  in  soil  which  is  always  damp.  The  ground 
wire,  which  is  best  nfade  of  flexible  copper  strip  having 
a  cross  section  not  less  than  that  of  t  inch  round  wire, 
should  be  carefully  soldered  and  riveted  to  this  plate, 
the  connection  from  the  arrester  to  ground  being  made 
as  short  and  as  direct  as  possible.  Where  there  are  metal 
flumes,  pipes,  or  rails,  it  is  advisable  to  rivet  and  solder 
the  ground  wires  to  them  in  addition  to  the  connections 
to  the  copper  plates. 

Choke   Coils.  —  Owing   to   the   oscillatory   character   of 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION.  289 

the  high-potential  discharge,  it  would  seem  as  if  a  choking 
coil  placed  between  the  arrester  and  the  apparatus  to  be 
protected  would  offer  such  resistance  to  the  discharge  as 
always  to  force  it  through  the  arrester  and  thence  to 
ground.  In  actual  service  it  has  been  found  that  the 
choke  coil  does  not  always  seem 
to  have  this  effect,  at  least  to  a 
marked  degree,  and  for  this  reason 
its  usefulness  has  been  questioned. 
It  is,  nevertheless,  an  inexpensive 
adjunct,  and  since  it  seems  to  pre- 
sent no  features  of  disadvantage, 
and  may  be  of  assistance  under  Fig.  165. 

certain  conditions,  engineers  are 

at  present  inclined  to  favor  its  use.     A  typical  choke  coil 
for  use  on  a  high-tension  line  is  illustrated  in  Fig.  165. 

Insulators.  —  A  desirable  insulator  for  line  use  should 
have  the  following  qualifications,  which  are  set  down  in 
the  approximate  order  of  their  importance. 

1.  A  large  dry  creepage  surface. 

2.  High  resistance  to  electrical  puncture. 

3.  High  mechanical  strength. 

4.  Ability  to  withstand  the  action  of  the  elements  with- 
out roughening  or  cracking. 

5.  Freedom   from  charring,  softening,  or   other  change 
of  physical  properties  under  continued  electrical,  mechani- 
cal, or  thermal  stress. 

6.  Ease  of  handling  and  transportation. 

7.  Reasonably  non-fragile  properties. 

8.  Comparative  cheapness. 

Porcelain  satisfies  more  of  these  conditions  than  does 
any  other  commercial  product,  and  hence  is  the  material 


290     POLYPHASE  APPARATUS  AND  SYSTEMS. 

most  commonly  employed.  When  used  for  insulators  it 
should  be  thoroughly  vitrified  and  homogeneous.  The 
material  should  be  non-absorbent  of  moisture,  and  not 
dependent  for  its  insulating  properties  on  the  surface 
glazing,  the  function  of  which  is  merely  to  give  a  smooth 
exterior  so  that  dust  or  moisture  may  not  be  retained  on 
the  surface.  While  the  general  character  of  a  porcelain 
insulator  may  be  roughly  gauged  by  breaking  it  and 
examining  the  appearance  of  the  fracture,  and  by  deter- 
mining the  absorptive  properties  of  the  broken  surface,  it 
is  essential,  in  order  to  verify  the  actual  insulating  strength, 
to  subject  each  insulator  to  a  high-potential  test. 

This  test  is  conveniently  made  by  placing  a  number  of 
inverted  insulators  in  a  metal  pan  which  is  filled  with  brine 
deep  enough  to  immerse  the  insulators  up  to  the  point  where 
the  tie  wire  is  to  be  attached.  The  brine  solution  is  also 
poured  into  the  pin  holes  of  the  insulators  up  to  the  depth  of 
the  threading.  A  series  of  metal  rods,  connected  electrically 
together  and  to  one  side  of  the  testing  circuit,  dip  into  the 
brine  in  the  pin  holes,  and  the  other  side  of  the  circuit  is 
connected  to  the  pan  in  which  the  insulators  are  contained. 
Application  of  the  test  pressure  is  now  made  for  the  desired 
time,  usually  four  to  five  minutes,  at  double  the  working 
voltage  on  which  the  insulator  is  to  be  used.  Any  insula- 
tors that  are  not  perfect  will  usually  fail  during  the  first 
minute  of  the  test,  the  puncture  being  manifested  by^  a 
shower  of  bright  sparks. 

In  addition  to  the  mode  of  testing  previously  described, 
a  so-called  "dry"  test  is  sometimes  made  by  mounting  the 
insulator  in  its  normal  upright  position  on  a  metal  pin,  a 
short  length  of  line  wire  being  attached  to  the  top  groove 
in  the  regular  way.  The  test  pressure  is  then  applied 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION.  2QI 

between  the  pin  and  the  wire.  An  important  modifica- 
tion of  the  preceding  is  to  subject  the  insulator  during  this 
test  to  a  brisk  spray  of  water,  simulating  a  heavy  rain  storm. 
This  is  known  as  the  wet  test,  and  the  standard  conditions 
usually  call  for  a  water  precipita- 
tion equivalent  to  a  one-fifth  inch 
per  minute  rain  fall,  the  direction 
of  the  spray  being  at  45  degrees 
from  the  vertical.  This  test  gives 
an  indication  of  the  ability  of  the 
insulator  to  maintain  the  under 
surfaces  in  a  dry  condition. 

A  good  margin  of  safety  is 
usually  found  in  any  insulator  that 
will  withstand  for  fifteen  minutes 
a  dry  test  of  three  times  the  nor- 
mal working  voltage.  The  crite- 
rion under  the  wet  test  is  rather 
indefinite,  owing  to  the  uncertain 
effect  of  air  currents,  splashing, 
and  other  factors,  but  may  be 
placed  at  about  one  and  one-half 
times  normal  voltage  for  a  fifteen- 
minute  period. 

Fig.  166. 

Fig.    1 66   shows    one    form   of 

high-tension  porcelain  insulator  mounted  on  its  iron  pin. 
This  insulator,  made  by  the  Ginori  Company  of  Italy,  is 
tested  at  60,000  volts,  and  is  used  on  a  3o,ooo-volt  trans- 
mission in  India.  It  is  8j  inches  in  height,  6J  inches  in 
diameter  at  the  widest  part,  and  weighs  7  pounds,  the  en- 
tire insulator  being  molded  in  one  piece. 

The  type  developed  by  American  engineers  and  used  on 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


most  of  the  notable  high-tension  transmission  systems,  con- 
forms to  the  general  design  shown  in  Fig.  167,  which  illus- 
trates the  principal  features  of  an  insulator  manufactured  by 
the  Locke  Company.  It  is  built  up,  as  seen,  of  four  separate 
shells  cemented  together,  and  is  suited  for  use  on  circuits 

of   60,000    volts.       The 

•f ~l^iH^r~Vl  test  pressure  is   120,000 

volts  for  five  minutes  on 
the  completed  insulator, 
the  separate  shells  being 
individually  tested  at  an 
average  of  50,000  volts 
each  for  two  minutes. 
The  principal  dimen- 
sions appear  in  the  cut, 
and  the  net  weight  is 
about  25  pounds. 


L. 


Jig.  167. 


This  general  type  of  insulator,  using  from  two  to  four  or 
even  five  separate  shells,  according  to  the  conditions,  is 
considered  by  American  engineers  and  manufacturers  to  be 
preferable  to  the  one-piece  type  for  work  at  about  15,000 
volts  and  over.  The  individual  shells  being  relatively 
small  and  light,  it  appears  easier  to  secure  a  homogeneous 
and  reliable  structure  in  this  way  than  if  it  were  attempted 
to  mold  in  one  piece  a  single  block  of  porcelain  of  the 
intricate  shape  and  ample  dimensions  necessitated  at  the 
higher  voltages. 

While  porcelain  has  been  used  for  insulators  to  the  prac- 
tical exclusion  of  all  other  materials,  the  use  of  glass,  which 
has  many  of  the  desirable  characteristics  of  the  former 
material,  has  had  a  considerable  application,  especially  for 
circuits  under  10,000  volts.  In  some  cases,  glass  insula- 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION.   293 

tors  have  been  successfully  used  on  very  high  voltages, 
notably  in  the  installation  at  Provo,  Utah,  which  transmits 
at  40,000  volts.  Nevertheless,  when  in  massive  shapes, 
glass  appears  somewhat  difficult  to  anneal,  and  hence  is  not 
as  strong  mechanically  as  desirable,  nor  as  well  able  to 
resist  extremes  of  temperature.  These  disadvantages  have 
appeared  to  militate  against  any  extensive  use  of  glass, 
which  has  had  only  a  limited  application  up  to  the 
present. 

Line  Construction.  —  The  great  majority  of  transmission 
lines  are  carried  on  wooden  poles  with  one  or  two  circuits 
per  pole,  reference  being  had  to  three-phase  transmissions 
only,  since  other  systems,  which  require  a  greater  weight  of 
copper,  are  used  only  in  exceptional  cases,  or  for  local 
distribution  circuits.  Where  the  pole  line  carries  a  single 
circuit,  the  wires  are  preferably  arranged  in  an  equilateral 
triangle,  to  equalize  the  inductance  of  each  phase,  one  wire 
being  carried  at  the  top  of  the  pole  so  that  but  one  cross- 
arm  is  required.  In  the  case  of  two  circuits  per  pole,  four 
of  the  wires  may  be  carried  on  one  cross-arm  at  the  top,  and 
the  remaining  two  on  a  second  cross-arm  beneath.  With 
this  arrangement  the  wires  of  each  circuit  are  spaced  in  a 
triangle  as  before,  except  for  having  the  apex  downward,  — • 
an  arrangement  which  facilitates  the  lineman's  access  to 
the  wires  from  below.  The  poles  are  of  such  a  length  that 
the  lowest  line  wires  shall  be  at  least  20  feet  above  the 
ground  at  the  middle  of  the  span.  The  distance  between 
poles  depends  largely  on  the  number  and  size  of  conductors 
carried,  and  on  the  size  of  pole.  Good  practice  establishes 
about  the  following  averages: 


294 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Pole  Spacing. 


No.  of 
Conductors. 

Size  of  Conductors,    B.  &  S. 

Top  Dia. 
of  Pole. 

Spacing  of 
Pole. 

No.  of  Poles 
per  Mile. 

3 

Nos.  6-1 

7  in. 

150  ft. 

35 

6 

Nos.  6—i 

7  in. 

150  ft. 

35 

3 

Nos.  0-4/0 

8  in. 

125  ft. 

42 

6 

Nos.  0-4/0 

8  in. 

I  10  ft. 

48 

3 

250,000-400,000  cm. 

9  in. 

100  ft. 

53 

6 

250,000—400,000  cm. 

10  in. 

Soft. 

66 

The  above  values  are  for  straightaway  work,  and  closer 
spacing  is  necessary  on  curves. 

The  distance  between  line  wires  depends  mainly  on  the 
voltage.  The  table  below  gives  desirable  spacings: 


Line  Voltage. 


Conductor  Spacing. 


2,000-  6,000  incl. 

2  ft.   4  in. 

10,000-20,000  incl. 

3  ft.   4  in. 

20,000-30,000  incl. 

4  ft.   o  in. 

30,000—40,000  incl. 

5  ft.   o  in. 

40,000-50,000  incl. 

5  ft.  o  in. 

50,000-60,000  incl. 

6  ft.   o  in. 

During  the  last  few  years  steel  towers  have  been  exten- 
sively used  for  supporting  the  conductors  on  the  longer 
transmission  systems,  especially  those  at  40,000  volts  and 
over.  This  construction  is  deemed  to  afford  several  impor- 
tant advantages,  chief  among  which  are  strength  and 
permanence  due  to  the  absence  of  the  decay  which  ulti- 
mately weakens  and  destroys  wooden  poles.  Other  advan- 
tages are  that  metal  towers  are  not  subject  to  injury  from 
lightning,  or  to  burning,  and  that  by  reason  of  the  much 
longer  spans  that  may  be  used,  the  number  of  insulators, 
and  thus  the  number  of  points  where  trouble  may  occur, 
is  reduced  to  a  minimum.  The  general  design  shown  in 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION.  2Q5 

Fig.  1 68  is  illustrative  of  the  structures  referred  to,  and  shows 
a  tower  built  up  of  light  but  strong  angles  and  channels, 
securely  cross-braced.  This  tower  carries  two  three-phase 
circuits  spaced  6  feet  between  conductors.  It  is  17  feet 
square  at  the  base,  Do  feet  high  over  all,  and  measures  50 
feet  to  the  lowest  conductor.  The  net  weight  is  4,000 
pounds. 

The  towers  are  shipped  knocked  down  and  are  assembled 
in  the  field.  Towers  of  this  general  design  are  used  on  the 
i6o-mile  transmission  of  the  Mexican  Light  and  Power 
Company,  to  Mexico  City  and  to  El  Oro,  and  on  most  of 
the  other  important  long-distance  lines  installed  during  the 
last  few  years. 

In  order  to  secure  the  maximum  immunity  from  inter- 
ruptions, a  duplicate  pole  line  is  sometimes  installed  with 
one  or  two  circuits  per  pole,  as  the  case  may  be.  In  this 
event,  whether  the  transmission  line  is  erected  on  wooden 
poles  or  on  steel  towers,  the  pole  lines  are  separated  by  a 
space  depending,  on  the  height  of  the  pole,  so  that  damage 
to  one  line  may  not  be  communicated  to  the  other.  In 
transmissions  of  this  character,  whether  using  a  single  or 
a  double  pole  line,  a  wide  space  is  cleared  on  either  side 
so  as  to  protect  the  lines  from  injury  by  falling  trees.  For 
this  purpose  power  companies  usually  acquire  control  of 
a  strip  of  land  of  the  necessary  amount,  the  right  of  way  for 
the  duplicate  steel  tower  lines  from  Niagara  Falls  to  Toronto 
being,  for  example,  80  feet. 

In  the  case  of  wood  pole  lines  writh  maximum  spans  of 
about  150  feet,  soft-drawn,  solid  wire  is  generally  used  for 
the  line  conductors  where  the  copper  cross  section  is  not 
too  great  to  prevent  ease  of  handling.  In  the  case  of  steel 
tower  lines,  where  spans  of  400  to  600  feet  are  commonly 


296     POLYPHASE  APPARATUS  AND  SYSTEMS. 

employed,  stranded  conductors  of  hard-drawn  copper  are 
preferably  used,  as  these  have  about  50  per  cent  greater 


Fig.  168. 

tensile  strength  for  the  same  cross  section,  besides  the  advan- 
tage of  flexibility  and  greater  ease  in  handling.     It  is  con- 


LIGHTNING  PROTECTION  AND  LINE  CONSTRUCTION.  297 

sidered  advisable  to  string  an  additional  conductor,  usually 
of  galvanized  iron,  above  the  transmission  line.  This 
additional  conductor  is  grounded  at  both  ends  of  the  line 
and  at  frequent  intervals  along  the  line,  and  appears  to  be 
of  much  assistance  in  obviating  troubles  from  lightning.  In 
Fig.  1 68,  a  ground  wire  of  this  sort  is  carried  on  a  standard 
which  is  seen  at  the  top  of  the  pole  between  the  two  line 
circuits. 

Where  parallel  transmission  lines  are  run  on  the  same  or 
on  adjacent  poles,  it  is  customary  to  transpose  the  conduct- 
ors at  intervals,  in  order  to  minim' ze  the  effect  of  mutual 
induction  between  the  separate  circuits  and  to  reduce  the 
inductive  effect  of  the  power  wires  on  telephone,  telegraph, 
or  other  power  circuits  in  the  vicinity.  Transposition  is 
effected  by  changing  the  relative  position  of  the  several  line 
wires  so  that  the  wire  carrying  phase  No.  i,  for  instance, 
will  be  first  at  the  top,  then  at  the  lower  right-hand  corner, 
and  at  a  further  point  at  the  lower  left-hand  corner  of  the 
triangle.  One  or  more  complete  spirals  are  made  in  this 
way  throughout  the  length  of  the  transmission,  according 
to  the  conditions. 

Telephone  lines  for  intercommunication  between  stations 
are  ordinarily  carried  on  the  same  poles  that  support  the 
transmission  wires,  and  must  be  transposed  at  frequent 
intervals,  or  the  strong  induction  from  the  power  circuits 
will  entirely  prevent  conversation.  Owing  to  the  high 
potentials  which  are  sometimes  accidentally  communicated 
to  the  telephone  wires  by  induction  or  by  contact,  the  practice 
has  recently  been  inaugurated  of  connecting  the  telephone 
instruments  to  the  line  through  a  small  highly  insulated 
transformer  having  a  i :  i  ratio,  so  that  no  dangerous  poten- 
tials may  get  through  to  the  telephone  terminals. 


298     POLYPHASE  APPARATUS  AND  SYSTEMS. 


CHAPTER   XII. 
TWO-PHASE   SYSTEM. 

Polyphase  Systems  and  Combinations.  —  Any  arrange- 
ment of  conductors,  carrying  two  or  more  single-phase 
alternating  currents,  having  a  definite  phase  relation  to 
each  other,  constitutes  a  polyphase  system.  The  systems 
commonly  employed  for  the  generation  and  distribution  of 
power  by  polyphase  currents  are  the  two-phase  and  the 
three-phase  systems. 

Polyphase  currents  are  usually  produced  by  alternators 
the  armatures  of  which  are  so  wound  that  the  electro- 
motive forces  at  the  terminals  correspond  to  the  number 
of  phases,  and  arrive  at  a  maximum  in  a  fixed  and  definite 
relation  to  one  another. 

In  the  two-phase  system  the  two  electromotive  forces 
and  currents  are  90  degrees,  or  one-fourth  of  a  cycle,  apart. 
The  relations  of  the  curves  to  each  other,  and  their  instan- 
taneous values,  can  be  seen  from  the  development  of  the 
diagram  of  single  harmonic  motion  (Fig.  169).  The 
maximum  of  one  wave  occurs  when  the  value  of  the  other 
is  zero.  If  the  pressure  in  any  one  of  the  coils  Oa  or  Ob  is  i, 
the  pressure  between  the  ends  ab  is  v7  2  --=  1.414. 

The  windings  of  a  polyphase  machine  may  be  combined 
in  a  number  of  ways,  each  affecting  the  relation  of  the 
electromotive  forces  of  the  outside  conductors,  as  shown 
in  Figs.  170  to  173.  These  diagrammatically  represent 


TWO-PHASE    SYSTEM. 


299 


the  coils  of  a  two-phase  machine,  in  which  the  electro- 
motive forces  may  be  considered  as  being  either  generated 
or  absorbed.  In  Fig.  170  all  the  coils  are  in  series,  form- 


1180° 


270-° 


Fig.  169. 

ing  a  continuous  winding,  tapped  at  four  points.     Leads 

i  and  2  constitute  the  circuit  of  one  phase,  and  3  and  4 

that  of  the  second  phase.     The  E.M.F.  between  the  wires 

of  different  phases  is  1.414  -*-  2  times  that  between  leads 

of  the  same  phase.     In  Fig.  171  the  windings  of  each  phase 

are  separate.     This  arrangement  can  be  made  interlinked 

by  joining  the  two  circuits  where  they  cross,  thus  forming 

a  common   center,  as 

shown    in     Fig.    172. 

The  relation  of  E.M.F. 

is  the  same  as  in  Fig. 

171.    The  grouping  of 

coils,    shown   in   Fig. 

171,  may  also  be  made 

interlinked  by  joining 

leads  4   and    2    (Fig. 

173),  which  become  a 

common  return  for  i 


Fig.  170. 


and  3.  The  E.M.F.  between  the  two  outgoing  wires  is 
1.414  times  that  between  each  outgoing  wire  and  the 
common  return. 


3°° 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


The  windings  of  interlinked  systems  are  classed  accord- 
ing to  their  connections  as  "  Ring,"  or  "  Star."  Figs.  170 
and  172,  respectively,  show  the  ring  and  star  connections  of 
the  two-phase  system. 

In  the  three-phase  system,  the  star  and  ring  connections, 
respectively,  are  usually  designated  as  Y  and  A  (Delta), 
from  their  resemblance  to  these  symbols.  The  winding 


connections  of  most  commercial  two-phase  machines  are 
Interlinked. 

.No  matter  what  the  arrangement  of  the  winding  may 
be  in  a  polyphase  machine,  whether  the  coils  are  inter- 
linked, or  separately  grouped,  ring  or  star  connected,  the 
principles  of  action  are  the  same,  and  the  characteristic 
polyphase  results  are  equally  present. 

Polyphase  systems  have  two  desirable  features:    First, 


TWO-PHASE    SYSTEM.,  |; 


301 


the  supply  of  power  is  continuous  and  uniform,  thus 
increasing  the  capacity  of  apparatus,  and  in  some  systems, 
that  of  transmission  conductors;  and,  second,  the-  use  of 
revolving  types  of  induction  apparatus  is  permitted,  which 
do  not  require  any  form  of  moving  contacts. 

Transformer  Connections.  —  A  number  of  combinations 
of  two-phase  circuits  can  be  made  by  suitably  arranging 


lO'OO 


\OOQ 


i  f 

2 
4 

Fig.  174 


1  <^ 

i  qoo       A        <f 

3  < 


Jo 


1000       B 


Fig.  175. 

transformers  with  due  regard  to  the  generator  windings. 
Fig.  174  shows  the  connections  commonly  used  for  light- 
ing and  transmission  of  power.  The  arrangement  consists 
of  two  single-phase  transformers,  the  phases  being  sepa- 
rated in  both  primary  and  secondary.  Two  of  the  sec- 
ondary leads  are  sometimes  joined  (Fig.  175),  making  a 
common  return  for  the  other  wires.  The  two  circuits  being 
90  degrees  apart,  the.  voltage  between  i  and  4  is  V '2  times 
that  between  the  outside  wires  and  the  common  return,  and 


302 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


the  current  in  the  common  return  is  V72  times  that  in  each 
of  the  outers.  This  arrangement  is  best  adapted  for 
supplying  current  of  minimum  potential  to  apparatus  in 
the  vicinity  of  the  transformers.  It  is  more  frequently  used 
in  connection  with  motors  operating  from  the  secondaries 
of  the  transformers.  Fig.  176  shows  another  arrangement 
of  transformers  where  the  common  return  is  used  on  both 
primary  and  secondary.  As  will  be  explained  farther  on, 
this  connection  is  permissible  only  when  the  power  of  the 


1    1 

<r 

1 

^^ 

A 
! 

io!oo 

1 

3       ! 

* 

2 

14OO 

^ 

B 

10,00 
I 

1 

4      1 

•*• 

<; 

Fig.  176. 

two  circuits  is  consumed  by  one  unit,  or  when  both  sides  of 
the  system  are  balanced. 

Two-Phase  to  Three-Phase.  —  It  is  possible,  by  a  combi- 
nation of  two  transformers,  to  change  one  polyphase  sys- 
tem into  any  other  polyphase  system.  The  transformation 
from  two-phase  to  three-phase,  or  vice  versa,  is  effected 
by  proportioning  the  windings  as  shown  in  Fig.  177,  com- 
monly called  the  Scott  connection.  One  transformer  is 
wound  with  a  ratio  of  transformation  of  1,000  to  100;  the 
other  with  a  ratio  of  1,000  to  86.7.  The  secondary  of  this 
transformer  is  connected  to  the  middle  of  the  secondary 
winding  of  the  first.  In  Fig.  178,  A  B  represents  the  second- 
ary volts  from  A  to  B  in  one  transformer,  called  the  main 
transformer.  At  right  angles  to  AB  the  line  CO  represents, 


TWO-PHASE    SYSTEM. 


303 


in  direction  and  quantity,  the  pressure  O  to  C  of  the  second 
transformer,  called  the  teaser.  From  the  properties  of  the 
triangle  it  follows  that,  at  the  terminals  A,  B,  C,  three  equal 
pressures  will  exist,  each  differing  from  the  others  by  120 
degrees,  and  giving  rise  to  a  three-phase  current. 

For  this  trans- 
formation on  a 
small  scale,  a 
sufficiently  close 
approximation 
is  secured  by 
the  use  of  stand- 
ard transform- 


f 

5   |          ~ 

io|oo 

<  >  —  i  T 

•i. 

<     >    oJ       i 

1C 

0 

T 
io'oo 

1    IsJ.r 

100       . 

^ 

<  •  s    i             M 

6  i  -C 

Fig.  177. 
ers,  the  teaser  having  a  ratio  of  10  to   i,  and 


the  main 
transformer  a  ratio  of  9  to  i. 

The  current  in  the  winding  AB,  being  a  resultant  of  the 
other  two  phases,  is  greater  than  if  the  change  to  three- 
phase  were  not  made;  and,  consequently,  for  the  same 
heating,  necessitates  a  larger  cross 
section  of  copper  in  the  secondary. 
This  current  being  15  per  cent  greater 
than  in  a  normal  single-phase  trans- 
former, the  secondary  copper  must  be 
larger  in  the  ratio  of  i.i$2  to  i.oo2,  or 
an  increase  of  32  per  cent.  This 
means  that  the  main  transformer, 
considering  he  total  material  in  its 

construction,  is  the  equivalent  of  a  transformer  of  about  8 
per  cent  greater  output.  Taking  both  transformers  together 
it  is  seen  that  the  two-phase,  three-phase  transformation 
involves  the  equivalent  of  4  per  cent  additional  capacity. 
If  the  two  transformers  are  made  interchangeable  the  teaser 


I-OO-<Z- 


Fig.  178. 


304 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


must  be  as  large  as  the  main '  transformer,  and  the  total 
increase  amounts  to  8  per  cent.  The  secondary  of  each 
interchangeable  transformer  has  two  taps,  giving  50  per 
cent  and  86.7  per  cent  of  the  full  voltage,  so  that  either 
transformer  can  serve  as  the  teaser  by  using  the  proper 
terminals. 

In  the  long-distance  transmission  of  power  the  genera- 
tors are  sometimes  wound  two-phase,  and  the  secondary 
distribution  at  the  receiving  end  is  likewise  by  the  two- 
phase  system,  while  on  account  of  the  saving  in  copper 


Fig.  179. 

the  transmission  is  by  the  three-phase  system.  Such  is  the 
arrangement  of  the  apparatus  at  the  generating  end  of  the 
Niagara-Buffalo  plant.  The  distribution  in  Buffalo,  how- 
ever, is  mainly  by  the  three-phase  system.  Fig.  179  shows 
the  transformer  connections  for  changing  two-phase  to 
three-phase  and  back  again. 

Two-Phase  Four- Wire  System.  —  This  system  consists 
of  two  separate  circuits,  derived  from  two  armature  wind- 
ings in  quadrature  with  each  other,  which  may  be  either 
independent  or  interlinked,  or  from  a  continuous  armature 
winding  tapped  at  four  equidistant  points.  The  practical 
application  of  this  system  is  illustrated  in  Fig.  180.  Each 


TWO-PHASE    SYSTEM. 


305 


306     POLYPHASE  APPARATUS  AND  SYSTEMS. 

of  the  two  generators  A  and  B  delivers  two-phase  currents 
of  low  potential  to  the  step-up  transformers  RT,  RT1,  RT2, 
RT3,  through  the  switchboard  D.  The  transmission  lines 
L,  L1,  L2,  L3,  receive  and  transmit  current,  at  a  high  pres- 
sure, to  a  substation  conveniently  located  with  reference 
to  the  districts  where  lights  and  motors  are  to  be  supplied. 
The  high-potential  current  is  here  reduced  by  the  trans- 
formers LT,  LT1,  LT2,  LT3,  to  a  commercial  pressure 
suitable  for  local  distribution,  through  the  switchboard  F. 
Beginning  at  the  bottom  of  the  figure,  the  first  four-wire 
system  is  used  to  supply  alternating  current  to  the  rotary 
converter  MG,  which,  in  turn,  delivers  direct  current  at 
500  volts  to  a  trolley  line  operating  the  street-car  systems 
K.  The  second  circuit  supplies  the  motors  M,  M1,  M2, 
M3,  either  of  the  synchronous  or  induction  type.  The 
next  four-wire  system  is  divided  into  two  distinct  circuits, 
supplying  current  to  incandescent  lamps  through  the 
transformers  b,  b1,  b2.  The  next  circuit  supplies  current 
for  arc  lighting  through  a  motor  generator  MG2.  A  rotary 
converter  is  also  operated  from  the  last  circuit,  and  delivers 
low-voltage  current  for  electrolytic  purposes.  The  rotary 
converters  in  practice  are  supplied  with  transformers,  not 
shown  in  the  diagram,  which  deliver,  at  the  rotary  termi- 
nals, an  alternating  current  of  the  proper  voltage. 

The  two  single  circuits  must  be  balanced  as  nearly  as 
possible,  and  for  this  purpose  the  four  wires  must  be  car- 
ried through  the  same  district  to  be  supplied  with  power 
or  light.  In  order  to  obtain  economy  in  copper  in  a  sec- 
ondary system  of  distribution,  three-wire  mains  may  be 
used.  In  the  two-phase  four-wire  system,  where  motors 
are  to  be  supplied,  the  two  independent  three-wire  circuits 
must  be  brought  together,  making  six  wires  irj.  all, 


TWO-PHASE    SYSTEM. 


307 


The  measurement  of  power  by  this  system  is  obtained 
by  the  use  of  a  wattmeter  inserted  in  each  circuit,  as  in  a 
single-phase  system.  The  sum  of  the  two  readings  gives 
the  total  power  supplied.  In  a  balanced  system,  twice  the 
reading  of  one  wattmeter  will  give  the  power. 

Two-Phase   Three-Wire   System.  —  By  joining  any   two 


Fig.  181. 

conductors  in  the  four-wire  system,  a  common  return  is 
made  for  the  two  circuits.  This  arrangement  of  circuits 
is  called  the  two-phase  three-wire  system.  As  previously 
shown,  the  pressure  between  the  common  conductor  and 
the  others  is  41.4  per  cent  higher  than  that  which  existed 
before.  Since  the  current  in  the  common  conductor 
exceeds  by  this  same  percentage  the  current  in  the  outers. 


308     POLYPHASE  APPARATUS  AND  SYSTEMS. 

the  common  conductor  must  be  of  suitably  larger  cross 
section,  in  order  to  keep  the  loss  the  same. 

The  general  application  of  this  system  is  shown  in  Fig. 
181.  Two  terminals  of  the  generator  coils  are  united;  and 
the  three  leads,  forming  an  interconnected  two-phase  sys- 
tem, are  run  to  wherever  motors  and  lights  are  to  be  sup- 
plied. When  motors  are  used,  connection  is  made  directly 
with  the  main  leads,  or,  if  the  motors  are  wound  for  low 
voltage,  connection  is  made  through  two  transformers. 
The  motors,  which  are  of  the  ordinary  two-phase  type,  may 
have  their  terminals  connected  either  on  the  three-wire  or 
four-wire  system. 

Where  lights  are  supplied,  the  transformers  may  be  con- 
nected singly  to  only  one  circuit,  or  in  pairs  on  two  cir- 
cuits, with  a  common  return.  In  practice,  it  is  essential 
that  both  phases  be  equally  loaded. 

In  this  arrangement  of  conductors  there  is  an  unbalan- 
cing of  both  sides  of  the  system  on  an  inductive  load,  which 
exists  even  though  the  energy  load  is  equally  divided. 
This  unbalancing  is  due  to  the  fact  that  the  E.M.F.  of  self- 
induction  in  one  side  of  the  system  is  in  phase  with  the 
effective  E.M.F.  in  the  other  side,  thus  distorting  the  uni- 
form current-distribution  in  both  circuits. 

The  distribution  of  currents  and  electromotive  forces  in 
the  three  conductors  in  the  single-phase  three-wire,  the 
three-phase  and  the  two-phase  three- wire  system,  is  shown 
in  the  following  table.  The  figures  are  the  results  of  experi- 
ments to  determine  the  self-induction  of  underground 
tubes. 

The  voltage  per  circuit  is  always  equal  for  equal  loads  in 
the  single-phase  three-wire  and  in  the  three-phase  systems, 
but  the  induction  unbalancing  of  the  two-phase  three- 


TWO-PHASE    SYSTEM. 


309 


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3  CONDUCTORS. 

MAIN  TUBB. 

ngle-Phase  3  Wire 
hree-Phase  3  Wire 
wo-Phase  3  Wire 

c/5  H  H 

OF    THE 

UNIVERSITY 

OF 


3IO     POLYPHASE  APPARATUS  AND  SYSTEMS. 

wire  system  is  beyond  the  range  of  practical  operation. 
These  results  were  obtained  with  low-tension  systems  and 
moderate  drops.  The  unbalancing  effect  is  much  greater 
with  higher  voltage  and  drops.  The  four-wire  two-phase 
system  would,  of  course,  show  no  such  unbalancing. 


THREE-PHASE   SYSTEM. 


CHAPTER  XIII. 
THREE-PHASE  SYSTEM. 

Curves  of  E.M.F.  —  The  E.M.F.  impulses  in  a  three- 
phase  system  follow  one  another  at  intervals  of  1 20  degrees. 
The  instantaneous  values  and  the  relation  of  the  phases, 
developed  from  the  diagram  of  simple  harmonic  motion, 
are  shown  in  Fig.  182.  The  curves  a,  b,  c,  represent  the  elec- 
tromotive forces  produced  by  three  sets  of  generator  coils. 
If  in  the  circle  the  distance  from  O  to  a,  b,  and  c,  be  taken 


Fig.  182. 

equal  to  i,  it  follows  from  the  diagram  that  the  lines  join- 
ing a,  b,  and  c  are  equal  to  \/3  =  1.732.  That  is,  the 
pressure  between  the  ends  of  any  two  of  the  generator 
coils  in  a  three-phase  system  is  1.732  times  that  between 
the  common  juncture  O  and  the  terminals  of  the  coils. 

It  will  be  seen  from  the  diagram  that  each  one  of  the 
coils  successively  serves  as  a  return  for  the  other  two,  and 
that  the  algebraic  sum  of  the  currents  in  the  system,  is 


312 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


zero.  The  three-phase  system  may  be  resolved  into  three 
single  circuits,  with  a  common  or  grounded  return.  The 
sum  of  the  currents  being  zero,  no  current  will  flow  in  the 
return  conductor,  and  it  may  be  dispensed  with.  The 
system  then  becomes  the  ordinary  F-connected  arrange- 
ment. 

Transformer  Combinations.  —  The  ring  and  the  star  con- 
nections of  three-phase  windings  —  whether  of  armature  in 


PRIMARY 


SECONDARY 


Fig.  183. 

which  the  electromotive  forces  are  induced,  or  transformer 
or  motor  in  which  the  electromotive  forces  are  absorbed 
—  are  designated  by  the  symbols  A  (delta)  and  Y  respec- 
tively. Figs.  183  to  187  illustrate  the  various,  three-phase 
combinations  of  single-phase  transformers  in  practical 
operation.  Fig.  183  shows  A  connection  of  both  primary 
and  secondary  terminals  of  transformers,  having  a  ratio  of 
10  to  i.  Fig.  184  shows  three  transformers,  F-connected 
in  both  windings.  The  ratio  of  pressures  between  any 
two  corresponding  terminals  in  primary  and  secondary  is 


THREE-PHASE    SYSTEM. 


313 


the  same  as  in  the  A  arrangement.  The  individual  trans- 
formers thus  connected  have  fewer  turns  for  the  same 
voltage  than  when  A  connected,  which  is  an  advantage 
where  the  line  voltage  is  high.  The  drawback  to  this 


PRIMARY 


SECONDARY 


Fig.  184. 


PRIMARY 


SECONDARY 


185. 


connection  is  the  unbalancing  of  voltage  and  inequality  of 
heating  that  are  liable  to  take  place  unless  all  the  neutrals 
are  connected  to  each  other  and  to  the  neutral  of  the  gen- 
erator. Fig.  185  shows  a  combination  of  A  and  Y  con- 


314     POLYPHASE  APPARATUS  AND  SYSTEMS. 

nection,  the  primaries  of  the  transformers  being  connected 
A,  while  the  secondaries  are  C9nnected  F.  A  fourth  wire 
may  be  led  from  the  common  center  of  the  three  second- 
aries. The  pressure  between  this  neutral  and  any  one 

of  the  outside  wires  is  — -  of  the   pressure  between    the 

V3 

outside  wires.  This  arrangement  is  known  as  the  "  three- 
phase  four-wire  system,"  and  is  frequently  used  in  sec- 
ondary distributing  systems,  especially  abroad.  In  Fig. 
1 86,  the  primaries  are  connected  Y,  the  secondaries  delta. 

A  connection  is  sometimes^  made  up  of  two  transformers 
(Fig.  187)  instead  of  three,  this  arrangement  being 
usually  called  the  "  open  delta"  connection.  The  pres- 
sures between  all  three  terminals  are  equal,  that  from  the 
open  side  of  the  triangle  on  the  secondary  being  due  to  the 
E.M.F.  of  the  corresponding  phase  of  the  primary  acting 
through  the  two  transformers  in  series.  This  arrangement 
is  frequently  used  with  motors,  its  chief  advantages  being 
its  simplicity,  and  permitting  the  use  of  available  trans- 
formers when  the  motor  cannot  be  fitted  with  three  trans- 
formers of  exactly  the  capacity  wanted.  Its  disadvantage 
is  that  the  continuity  of  polyphase  working  is  destroyed  in 
case  of  damage  to  one  of  the  twp  transformers.  Another 
disadvantage  of  the  open  delta  arrangement  is  that  15  per 
cent  more  transformer  capacity  is  required  for  the  same 
energy  than  with  the  ordinary  delta  connection  using 
three  transformers.  This  disadvantage  may  not  be  so 
important  in  small  transformers,  which,  when  chosen  in 
standard  sizes,  may  be  amply  large  for  the  work,  but  must 
be  taken  into  account  in  any  cases  where  the  capacity  is 
accurately  figured. 

In  general,  the  combination  of  three  transformers  with 


THREE-PHASE   SYSTEM. 


315 


both  sides  connected  delta  is  most  convenient  and  desir- 
able, for  the  reason  that  an  accident  to  one  does  not  prevent 
a  continuance  of  the  service.  In  this  contingency  it  is 


PRIMARY 


SECONDARY 


Fig.  186. 


PRIMARY 


SECONDARY 


Fig.  187. 

necessary  that  the  load  be  reduced  by  about  one-third 
(by  42  per  cent,  strictly  speaking)  to  prevent  overloading 
the  remaining  two  transformers,  which  are  thus  reduced 
to  the  open  delta  connection. 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Six-Phase  Connections  of  Transformers.  —  This  con- 
nection is  much  used  with  rotary  converters  and  is  illus- 
trated by  the  diagrams  on  page  193  (Figs.  115-118  inclu- 
sive). It  may  be  derived  from  any  system  of  transformers 
connected  in  three-phase  and  having  two  separate  second- 
ary windings,  by  reversing  one  of  the  two  sets  of  sec- 
ondaries. The  simplest  method  is  the  so-called  diametri- 
cal connection  which  is  described  under  the  preceding 
reference. 

Motor    Connections.  —  Motors    are    connected    to    the 


Generator 


secondaries  of  three  transformers  in  a  three-phase  system, 
as  shown  in  Fig.  188. 

The  primaries,  P1,  P2,  P3,  of  three  transformers  are 
connected  between  the  three  lines  A,  B,  C,  leading  from 
the  generator;  and  three  secondaries,  S1,  S2,  S3>  are  con- 
nected in  delta  to  the  three  lines,  a,  b,  c,  leading  to  the 
motor.  A  recording  wattmeter  of  the  three-phase  type, 
for  measuring  the  power  consumed  by  the  motor,  is  shown 
connected  in  the  system  with  the  field  spools  at  /,  the 
armature  circuit  a1  and  its  resistances  r,  between  the  three 
secondary  lines. 

Induction  motors  may  be  supplied  from  a  three-phase 


THREE-PHASE    SYSTEM. 


317 


generator  by  means  of  two  reducing  transformers  in  the 
manner  shown  in  Fig.  189.  This  arrangement  (which  is 
the  open  delta  connection)  is  identical  with  that  in  Fig. 
188,  except  that  one  of  the  transformers,  P3,  Ss,  is  left 
out,  and  the  two  other  transformers  are  made  correspond- 
ingly larger.  The  recording  wattmeter  is.  connected  in  the 


Generator 


Motor 


Generator 


<? 
Fig.  189. 

i*  " 

P 

1               i 

3V 

|s.'   " 

f     20J5V 

I  !'H 

§             c       T 
83           nU 

|     11p  V 

"ill 

d 
Fig.  190. 

secondary  circuit  in  the  same  way  as  in  the  use  of  three 
transformers. 

The  connections  of  three  transformers  for  a  low-tension 
distribution  system,  by  the  three-phase  four- wire  system, 
are  shown  in  Fig.  190.  The  three  transformers  have  their 
primaries,  P1,  P2,  P3,  joined  in  delta  connection,  and 
their  secondaries,  S1,  S2,  S3,  in  Y  connection.  Lines 
a,  b,  c,  are  the  three  main  three-phase  lines,  and  d  is  the 
common  neutral.  The  difference  of  potential  between  a 


3l8     POLYPHASE  APPARATUS  AND  SYSTEMS. 

and  b,  b  and  c,  and  a  and  c  is  200  volts,  while  that  between 
them  and  d  is  115  volts.  200-  volt  motors  are  joined  to  a, 
b,  and  c,  while  ii5-volt  lamps  are  connected  between  a 
and  d,  b  and  d,  or  c  and  d.  Line  d,  like  the  neutral  wire 
in  the  Edison  three-wire  system,  only  carries  current  when 
the  load  is  unbalanced. 

Measurement  of  Power.  —  In  a    F-connected   generator 

•p 

the  E.M.F.,  induced  in  each  phase,  is  —=-  and  the  energy 

E  V$> 

in  that  phase  is  7  X  —^,  E  being  the  E.M.F.  at  the  gen- 

V3 
erator     terminals.     In    a    delta-connected     generator    the 

current  in  each  phase  winding  is  —  ^r,  /  being   the   line 


current,  and  the  energy  is  £  X  —7=.     The  total  energy  for 

.V3 

the  three  phases,  in  the  cases  both  of  a  F  and  a  A  connected 
generator,  is  =  \/3  X  E  X  I.  This  formula  is  correct 
when  the  generator  output  is  of  a  non-inductive 
character.  If  a  phase  displacement  exists,  the  expression 
becomes  \/3  X  E  X  I  X  Cos  <£.  These  formulas  apply 
equally  well  for  determining  the  power  in  a  three-phase 
circuit,  irrespective  of  the  method  of  connections  of  the 
supplying  source  or  of  the  consuming  devices. 

As  an  illustration,  —  the  power  in  a  non-inductive  three- 
phase  circuit,  in  each  branch  of  which  100  amperes  are 
flowing,  the  voltage  between  lines  being  2,500,  is  found  as 
follows:  the  energy  in  each  phase  is  =  100  amperes  X  2,500 
volts  -s-  \/3  =  145  kilowatts,  and,  for  the  three  circuits,  is 
therefore  435  kilowatts.  If  the  circuit  had  a  power  factor 
of  80  per  cent,  the  energy  would  then  be  435  X  0.80  = 
348  kilowatts. 


THREE-PHASE    SYSTEM.  319 

The  power  supplied  by  three-phase  circuits  can  be  meas- 
ured by  the  use  of  three,  two,  or  one  wattmeter.  Three 
wattmeters  will  give  the  power  of  a  circuit  irrespective  of 
the  condition  of  balancing  or  lag.  The  sum  of  the  read- 
ings of  the  three  instruments  is  the  total  power.  Each 
wattmeter  must  be  connected  to  the  common  center  or 
neutral  of  the  system.  If  the  apparatus  is  connected 
delta,  it  is  necessary  to  make  an  artificial  neutral  with 
resistances.  Two  wattmeters  can  be  connected  so  that,  as 
long  as  the  power  factor  is  greater  than  50  per  cent,  the 
sum  of  thektwo  readings  equals  the  total  power.  The  dif- 
ference of  the  two  readings  will  give  the  power  when  the 
power  factor  is  less  than  50.  As  it  is  not  possible  to  tell 
when  the  power  factor  falls  below  this  point,  without 
reversing  the  connections,  this  method  is  inconvenient  and 
undesirable. 

When  three-phase  circuits  are  in  balance  in  respect  to 
load  and  power  factor  the  power  may  be  measured  by  one 
wattmeter.  Three  times  the  readings  of  the  single  watt- 
meter will  give  the  total  power  in  the  circuits.  Figs.  191 
and  192  show  the  connections  of  three-phase  recording 
wattmeters  for  a  low  and  for  a  high  voltage  circuit.  The 
wattmeter  is  provided  with  resistances,  r,  r  and  r1,  for 
creating  an  artificial  neutral.  The  armature  windings  are 
in  series  with  r1,  so  that  r1  +  a  =  r.  The  wattmeter,  dia- 
grammatically  illustrated  in  Fig.  191,  is  adapted  for  cir- 
cuits of  550  volts  and  less.  Fig.  192  shows  the  connection 
of  a  wattmeter  for  circuits  of  from  1,000  to  3,000  volts. 
Station  transformers  /  and  t  are  required  to  reduce  the 
pressure  for  the  armature  windings. 

The  connections  for  an  indicating  wattmeter  are  the 
same  as  those  for  a.  recording  wattmeter.  The  main  cur- 


320     POLYPHASE  APPARATUS  AND  SYSTEMS. 

r 


To  Generator 


To  Generator 


110-220-550  Volts 
Fig.  191. 


^/VWVltl 


11 50-2300  Volts 


To  I/TIC 


THREE-PHASE    SYSTEM. 


321 


rent  is  taken  by  the  stationary  or  low-resistance  coil,  while 
the  pressure  coil  is  of  high  resistance,  and  connected  to 
the  artificial  neutral. 

The  power  in  unbalanced  three-phase  circuits  is  meas- 
ured by  a  special  form  of  wattmeter  of  the  induction  type. 
This  wattmeter  is  also  largely  used  in  balanced  circuits. 


Fig.  193. 


Three-Phase  Circuits.  —  The  general  arrangement  of 
circuits  for  a  local  distribution  of  light  and  power  is  shown 
in  Fig.  193.  The  generators  are  wound  for  2,000  volts, 
feeding  direct  into  the  mains.  Step-down  transformers 
reduce  the  power  to  100  volts  for  lights  and  200  volts  for 
motors.  In  one  arrangement  alternating  inclosed  arc 
lights  are  shown,  operated  from  a  transformer.  A  200- 
volt  motor  is  supplied  by  three  transformers,  constituting 


322     POLYPHASE  APPARATUS  AND  SYSTEMS. 

a  system  of  secondary  mains.  In  the  second  arrangement 
a  motor  running  from  two  transformers,  and  a  general  dis- 
tributing system,  are  shown.  The  general  practice  is  to 
wind  the  generators  for  about  2,200  or  2,300  volts  at  full 
load  and  use  transformers  reducing  to  115  volts  for  light 
and  small  motors.  Where  secondary  mains  are  employed 
the  motor  pressure  is  200,  220,  440,  or  550  volts. 

Where  lights  and  motors  are  located  a  considerable  dis- 
tance from  the  generators,  the  cost  of  copper  is  reduced  by 
employing  transformers  to  raise  the  current  pressure.  An 
arrangement  of  three-phase  circuits  for  transmitting  power 
over  long  distances  is  shown  in  Fig.  194.  The  generator, 
direct-connected  to  the  source  of  power,  a  water-wheel,  is 
shown  at  A.  B  is  a  bank  of  step-up  transformers,  raising 
the  voltage  to,  say,  20,000.  As  this  voltage  is  higher 
than  can  be  used  with  any  apparatus  for  direct  utilization 
of  the  current,  step-down  transformers  Clt  C2,  and  C3,  are 
required. 

The  main  substation  contains  the  transformers  Cl  and 
C2.  This  is  a  true  central  or  distributing  station.  From 
this  point  the  distributing  feeders  are  taken  out  at,  say, 
2,000  volts,  for  the  commercial  primary  circuits  and  through 
the  bank  C2  at  115  volts,  to  feed  a  low-tension  network. 
Through  the  transformer  Cit  a  current  of  2,000  volts  is 
fed  direct  into  a  synchronous  motor,  and  into  transformers 
reducing  to  115  volts  for  supplying  motors  and  lights. 
The  substation  transformers  C2  furnish  current  for  a 
general  lighting  and  motor  service  at  /,  /,  and  H .  The 
voltage  for  this  distributing  system  is  controlled  by  the 
regulators  G. 

At  C3  another  bank  of  step-down  transformers  is  located. 
An  alternating  current  of  suitable  voltage  is  delivered  to 


THREE-PHASE   SYSTEM. 


323 


the  rotary  converter  Z>,  which  supplies  continuous  current 
to   the   electrolytic   vats  or  storage  battery  E.      A  rotary 


converter  might  also  furnish  direct  current  for  electric  rail- 
way service.  The  arrangement  of  apparatus  in  a  modern 
rotary  converter  substation  for  railway  service  has  already 
been  illustrated  in  Figs.  149  and  150. 


324 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Three-Phase  Lighting  Circuits.  —  For  both  arc  and 
incandescent  lighting  by  low-tension  three-phase  distri- 
bution, two  systems  are  in  use.  The  first  is  known  as  the 
single-phase  three-wire  system,  the  general  arrangement 
and  connections  of  which  are  shown  in  diagram  195.  The 
second  system  is  the  three-phase  four- wire,  which  has  been 


THREE-PHASE  DYNAMO 


SERIES  ARC  CIRCUIT 


SINGLE-PHASE  LIGHTING  FEEDER 


LIGHTING  TRANSFORMER 


u  u 


VWWVVNAV 


FOR  POWER  WIRE            f*Y"l                       j 

1 

SINGLE-PHASE      -•  
THREE  WIRE  SYSTEM 

..0... 

T 

L_ 
POWER  WIR 

GROUND 

LQ. 

i 

ip 

i 

-0- 

-0 
-0 

-o- 

-0- 

-o- 

-0- 
-0- 

ff 

Fig.  195. 

described    before.     The    connections    of    this    system   for 
lighting  installations  are  shown  in  diagram  196. 

In  the  first  system  the  incandescent  lighting  service  is 
mainly  supplied  from  one  circuit  of  the  three-phase  gene- 
rator, and  the  voltage  is  regulated  with  respect  to  the  needs 
of  the  three-wire  lighting  circuit.  The  primary  circuits 
having  a  small  drop,  make  it  possible  to  lay  out  a  secondary 
network  having  a  very  superior  regulation.  When  motors 


THREE-PHASE    SYSTEM. 


325 


are  operated  on  this  system,  a  separate  power  wire  is 
required,  the  connections  of  which  are  shown  in  the  dia- 
gram. 

The  advantage  of  the  four-wire  three-phase  system  is 
that  as  long  as  it  is  balanced,  the  generator  load  remains 
balanced.  Another  favorable  feature  is  that  its  outside 


X. 

i       i 

y^ 

1   1          1 

THREE-PHASE  DYNAMO 

BU8  BARS 
FEEDER 
) 

i 

6ER 

Ul  UJ  U 

1 

Jili 

ES  ARC  CIRCl 
BALANCED 

WWW 

178   EJ  ^  E 
nnn 

nm 

•>v\-« 

r 

I                    i 

i 

J 

i  1               i           T 

f 

._  i  i  i        -L1       ' 

_           III 

EEL 

O  —  ! 

-0 
-0 

•o- 
o- 

-o- 

o 

n 

1      ' 

LARGE  LIGHTING 
INSTALLATIONS 


SMALL  LIGHTING  _wp_p  DHA.C  urrrno          HOUSE  TO  HOUSE  TRANSFORMER 
INSTALLATIONS  THREE-PHASE  MOTOR   DISTR|BUT|ON  APPROXIMATELY  BALANCED 


Fig.  196. 


wires  are  available  for  operating  motors.  This  system  is 
not  so  desirable  in  cases  where  the  system  may  be  seri- 
ously unbalanced,  as  by  arc  lamps  or  other  very  inductive 
load  unequally  distributed.  It  is  also  inferior  in  simplicity 
and  in  facility  of  voltage  regulation  to  the  single-phase 
three- wire  system. 

Three-Phase  Generators.  —  The  three-phase  generator 
will  ordinarily  deliver  75  per  cent  of  its  rated  capacity,  in 
single-phase  current,  between  any  two  conductors,  with 
the  same  heating  as  when  delivering  full  three-phase  load. 


326 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


When  running  as  a  single-phaser,  or  when  the  load  is 
unbalanced  between  the  conductors,  the  potential  differ- 
ences of  the  phases  are  unequal.  The  phase  carrying  the 
load  will  have  one  voltage,  while  one  of  the  disused  or 


2080 


Main  Coil* 
Armature 


— H-— 


at«r  W ire 
Feeder 


Main  Coil 


Fig.  197. 

lightly  loaded  phases  will  have  a  higher  voltage  and  the  other 
a  lower  voltage.  In  machines  of  good  regulation,  these 
differences  are  small,  and  can  be  readily  taken  care  of  by 
regulators  in  the  feeder  circuits.  These  differences  are 
also  smoothed  out  by  the  use  of  motors  and  synchronous 


THREE-PHASE   SYSTEM. 


327 


apparatus  connected  to  the  three-phases.  The  unused 
phases  can  be  loaded  with  other  single-phase  currents,  thus 
giving  varying  degrees  of  unbalancing,  and  increasing  the 
load  with  normal  heating,  inversely  as  the  unbalancing,  up 
to  the  total  three-phase  capacity  of  the  generator.  As  a 
three-phase  generator,  for  a  given  output,  is  cheaper  and 
smaller  than  a  single-phaser,  it  will  often  be  found  desir- 
able to  install  a  three-phase  machine  for  single-phase 
working.  Intelligent  and  careful  arrangement  of  the 
feeder  circuits  will  give  the  best  possible  results  as  to 
regulation.  This  is  the  more  easily  obtained  by  the  use 
of  regulators,  which  modern  engineering  demands  shall 
be  installed  in  every  central  station. 

Monocyclic  System.  —  In  the  monocyclic  system,  which 
was  at  one  time  considerably  used,  the  generators  are  of 
a  special  type,  having  a  main  single-phase  winding  and  an 
auxiliary  or  teaser  winding  connected  to  the  central  point 
of  the  main  winding  and  in  quadrature  therewith.  The 
arrangement  of  these  windings  is  shown  diagrammatically 
;n  Fig.  197,  which  illustrates  also  the  disposition  of  the 
main  and  teaser  coils  in  their  respective  slots  on  the  arma- 
ture core.  The  teaser  coil  generates  a  voltage  equal  to 

about  25  per  cent  of 
that  of  the  main  coil, 
so  that  the  E.M.F. 
between  the  termi- 
nals of  the  main  coil 
and  the  free  end  of 

the  teaser  is  the  resultant  of  the  E.M.F.  of  the  two 
coils,  and  is  shown  in  magnitude  and  direction  by  Fig. 
198.  By  various  transformer  connections  it  is  possible 
to  resolve  this  triangle  of  E.M.F.  into  one  that  gives 


2080 

Fig.  198. 


328     POLYPHASE  APPARATUS  AND  SYSTEMS. 

a  practically  correct  three-phase  relationship  for  use  where 
polyphase  motors  are  to  be  used.  In  this  system  two 
wires  leading  from  the  ends  of  the  single-phase  wind- 
ing in  the  generator  supply  single-phase  current  to  the 
lighting  load,  a  third  wire  connected  to  the  end  of  the 
teaser  being  run  to  points  where  the  polyphase  motors  are 
installed. 


CHOICE   OF   FREQUENCY.  329 


CHAPTER    XIV. 
CHOICE   OF   FREQUENCY. 

High  Frequencies. —  In  designing  a  plant  for  the  distri- 
bution of  light  and  power  by  polyphase  currents,  one  of  the 
first  considerations  that  presents  itself  is  the  frequency 
of  the  apparatus.  Formerly,  the  frequencies  generally 
employed  in  the  United  States  were  125  and  133 
cycles,  or  15,000  and  16,000  alternations.  Abroad,  the 
commercial  frequencies  were  somewhat  lower,  varying  from 
80  to  100  cycles.  The  adherence  to  a  high  frequency  in 
this  country  for  over  ten  years  resulted  in  an  investment 
of  millions  of  dollars  in  this  particular  type  of  apparatus, 
and  made  the  introduction  of  new  types  of  lower  frequency 
into  old  and  existing  central  stations  somewhat  difficult, 
even  when  evident  economy  and  advantage  were  shown  to 
follow  upon  such  introduction. 

To-day  in  the  organization  of  a  new  plant,  the  problem  is 
almost  invariably  confined  to  the  selection  of  a  frequency 
of  60  cycles  or  under. 

There  were  frequently  strong  reasons  for  retaining  or 
adopting  125  or  133  cycles.  One  of  these  has  been  men- 
tioned above.  The  change  from  125  cycles  to  a  lower 
frequency  necessitated  a  complete  revamping  of  the  instal- 
lation, and,  with  the  exception  of  the  small  sizes,  the  trans- 
formers must  be  replaced.  Again,  when  a  low  first-cost 
of  a  plant  is  considered  of  more  importance  than  a  possible 


330     POLYPHASE  APPARATUS  AND  SYSTEMS. 

ultimate  saving  of  operating  expenses,  and  a  more  satisfac- 
tory service,  a  high  frequency  will  be  used.  The  gener- 
ators are  cheapen  The  transformers  are  also  smaller  and 
cheaper. 

One  of  the  drawbacks  to  the  use  of  high  frequencies, 
especially  in  the  transmission  of  power  over  lines  of  consid- 
erable length,  is  the  drop  of  voltage  due  to  the  reactance  of 
the  line,  which  increases  with  the  frequency.  For  illustra- 
tion :  the  reactance  of  1,000  feet  of  No.  i  wire,  at  25 
cycles,  is  0.0486  ohm,  and  at  125  cycles,  0.243  ohm.* 

By  reducing  the  frequency  from  1 2  5  cycles  to  2  5  cycles, 
in  the  above  case,  the  voltage  drop,  due  to  the  reactance 
and  resistance,  is  reduced  almost  one-half.  With  heavier 
conductors  and  higher  frequencies,  the  difference  is  still 
more  noticeable.  The  effect  of  frequency  on  the  voltage 
drop  in  transmission  lines  is  treated  at  further  length 
in  Chapter  XVI.  In  lighting  plants  employing  large 
conductors,  on  account  of  the  varying  power  factors  due 
to  changing  character  of  load,  the  irregularity  in  voltages 
at  high  frequencies  may  become  quite  marked.  As  we 
have  seen,  this  voltage  drop  is  not  all  energy  loss,  this  loss 
being  only  proportional  to  the  energy  component  of  the 
total  drop. 

Other  disadvantages  in  the  use  of  high  frequencies  are 
the  speed  at  which  both  generators  and  motors  must  run 
in  order  not  to  unduly  increase  the  number  of  poles,  and 
the  difficulty  in  connection  with  engine  regulation,  when  a 
number  of  generators  are  direct  driven,  and  operated  in 
parallel.  High-frequency  as  well  as  low-frequency  induc- 
tion motors  operate  better  at  high  speeds,  but  these  are 
undesirable  from  both  mechanical  and  commercial  stand- 

*  See  Table  of  Line  Constants  for  Power  Transmission,  p.  344. 


CHOICE    OF   FREQUENCY.  331 

points.  On  the  other  hand,  high-frequency  induction 
motors  of  reduced  speeds  have,  as  a  rule,  low  power  fac- 
tors. The  high-frequency  induction  motor  was  introduced 
to  meet  the  demands  for  motors  of  small  power  on  high 
frequency  circuits.  With  the  system  on  which  it  is  at 
present  operated,  it  may  in  time  become  a  thing  of  the 
past. 

To  sum  up :  High  frequencies,  i.e.,  over  60  cycles,  per- 
mit the  use  of  cheap  generators  and  transformers,  and,  in 
addition,  the  simple  and  satisfactory  operation  of  incan- 
descent and  arc  lamps.  They  have  the  disadvantage  of 
increasing  the  voltage  drop  and  idle  currents  of  circuits, 
with  consequent  bad  regulation  and  the  heating  of  the 
generator  at  light  loads,  of  not  permitting  the  parallel 
operation  of  direct-connected  machines  of  low  speed,  and 
the  further  disadvantage,  that  induction  motors  must 
either  run  at  excessive  speeds  or  with  poor  power  factors. 
Synchronous  motors  will  not  start  with  the  same  vigor  as 
on  lower  frequencies. 

Low  Frequencies.  —  Up  to  the  present  time  no  arc  lamp 
has  been  made  that  will  operate  with  entire  satisfaction  on 
frequencies  of  less  than  40  cycles.  Incandescent  lamps 
cannot  be  used  to  advantage  on  frequencies  less  than  30 
cycles.  Low-voltage  incandescent  lamps  show  no  flicker ; 
but  the  effect  of  fatiguing  the  eye  is  noticeable  at  25 
cycles,  especially  in  high-voltage  lamps. 

Transformers  are  somewhat  bulkier,  more  expensive,  and 
slightly  less  efficient  at  low  frequencies.  Induction  motors, 
while  likewise  larger  and  more  expensive,  as  a  rule  can  be 
built  with  equal,  if  not  better,  power  factors,  and  at  con- 
venient and  commercial  speeds.  Rotary  converters  can 
be  successfully  designed  for  60  cycles.  The  speed  is  high, 


332     POLYPHASE  APPARATUS  AND  SYSTEMS. 

however,  and  the  best  mechanical  and  electrical  results  are 
obtained  at  frequencies  under  40  cycles.  The  largest  use 
of  miscellaneous  power  by  rotary  converters  is  at  Niagara, 
where  a  frequency  of  25  cycles  is  employed. 

The  Niagara  plant  is  essentially  a  power  plant.  The 
use  of  current  for  both  arc  and  incandescent  lighting  is  of 
no  great  importance.  The  power,  electrically  generated 
on  a  scale  never  before  attempted,  is  used  locally  in  a  great 
variety  of  processes,  and  is  delivered  in  a  form  most  suit- 
able for  its  diverse  uses.  Power  by  the  direct-current 
system,  while  convenient  for  some  particular  operations, 
would  not  answer  equally  well  all  requirements  at  Niagara, 
and  would  be  unsuitable  for  long-distance  transmission. 
A  high-frequency  system  would  restrict  the  use  of  motors 
and  rotary  converters,  and  the  transmission  of  power  over 
very  long  distances.  Sixty  and  40  cycles,  however,  per- 
mitting the  general  use  of  lighting  apparatus,  do  not  give 
the  best  results  with  rotary  converters  of  large  output. 
The  operation  of  25-cycle  rotary  converters,  on  the  scale 
employed  at  Niagara,  shows  that,  for  the  purely  power 
conditions  there  existing,  this  frequency  was  wisely  chosen. 

A  frequency  of  25  cycles  is  also  used  by  the  Brooklyn 
Edison  Illuminating  Company  in  the  extension  of  their 
plant.  The  power  is  transmitted  within  an  area  covering 
75  miles,  to  various  substations,  where  25-cycle  rotary 
converters  are  stationed.  These  deliver  115  volt  direct- 
current  into  Edison  three-wire  mains.  The  Chicago 
Edison  Company  use  a  somewhat  similar  system  of  dis- 
tribution and  the  same  frequency. 

For  the  general  conditions  of  a  power  plant,  supplying 
alternating  current  for  induction  motors  and  lighting, 
and  making  a  specialty  of  furnishing  direct  current  on  a 


CHOICE    OF    FREQUENCY,  333 

large  scale,  at  some  distance  from  the  generating  plant, 
a  frequency  of  less  than  40  cycles  will  be  found  suitable. 

The  frequency  of  60  cycles,  or  7,200  alternations  per 
minute,  has  come  into  extensive  use.  In  Europe  the  fre- 
quency of  50  cycles  is  more  used  than  any  other.  These 
frequencies  have  the  advantage  of  considerably  reducing 
line  reactance  and  the  idle  currents  present  in  lighting 
systems  of  higher  frequencies.  They  are  adapted  for  the 
most  economical  results  in  a  general  distributing  system 
of  lights  and  motors.  On  account  of  the  good  regulation 
possible  with  these  frequencies  the  highest  economy  lamps 
can  be  used.  The  motors  are  excellent  in  respect  to 
efficiency  and  power  factor,  and  run  at  commercial  speeds. 
Both  motors  and  transformers  are  reasonable  in  cost. 

When  the  generating  units  are  direct  connected  to 
engines  of  extremely  slow  speed  and  operated  in  parallel, 
a  frequency  of  50  or  60  cycles  will  be  found  to  be  not 
desirable.  As  explained  in  Chapter  III,  the  permissible 
variation  in  rotative  speed  is  not  so  great  as  with  lower- 
frequency  generating  units. 

Choice  of  Frequency.  —  It  is  impossible  to  make  more 
than  the  most  general  application  of  the  foregoing  re- 
marks. Each  particular  case  must  be  studied  in  the  light 
of  its  special  conditions,  before  an  intelligent  decision  can  be 
made  as  to  the  proper  frequency  to  employ.  At  the  risk 
of  repetition,  the  following  general  recommendations  are 
suggested  as  embodying  the  latest  and  standard  practice : 

For  local  lighting  systems  with  incidental  demand  for 
power  in  small  units,  where  old  transformers  have  to  be 
retained,  and  where  a  cheap  plant  is  of  first  consideration, 
a  high  frequency  may  be  used,  but  should  be  discouraged 
as  much  as  possible. 


334  POLYPHASE   APPARATUS    AND    SYSTEMS. 

For  local  transmission  and  distribution  for  lighting  and 
power  purposes,  conditions  which  accompany  the  majority 
of  alternating-current  propositions,  a  standard  frequency 
of  60  cycles  can  be  used  to  advantage. 

In  power  and  lighting  plants,  supplying  current  to 
induction  motors  and  to  rotary  converters,  and  for  lighting, 
and,  finally,  for  very  long  transmissions  of  power,  a  fre- 
quency of  50  cycles,  or  thereabouts,  may  be  used.  This 
is  a  good,  all-round  frequency,  and  is  coming  into  more 
general  use.  It  is  much  employed  abroad. 

For  exclusively  power  plants,  where  lighting  is  of  no 
importance  whatsoever,  and  where  rotary  converters  and 
motors  of  large  size  or  slow  speed  are  to  be  supplied,  a 
frequency  of  25  to  30  cycles  may  be  used. 

Notwithstanding  the  opportunity  for  the  careful  exercise 
of  judgment  in  selecting  a  proper  frequency,  almost  equally 
good  results  can  be  obtained  with  widely  different  fre- 
quencies. As  an  illustration,  the  Brooklyn  Edison  Com- 
pany have  adopted  25  cycles  for  their  power  and  rotary 
converter  work.  The  Boston  Edison  Company  obtain 
practically  the  same  results,  using  a  frequency  of  60 
cycles.  Power  is  transmitted  over  the  150  miles  of  line 
of  the  Bay  Counties  Company  in  California  at  a  frequency 
of  60  cycles.  The  100  mile  transmission  of  the  Govern- 
ment of  Mysore,  India,  is  accomplished  at  a  frequency  of 
25  cycles. 


RELATIVE   WEIGHTS    OF    COPPER.  335 


CHAPTER     XV. 

RELATIVE    WEIGHTS  OF  COPPER   FOR   VARIOUS 
SYSTEMS. 

As  the  transmission  and  distribution  of  power  often 
involves  a  large  outlay  for  copper  conductors,  it  is  most 
important  to  ascertain  what  system  and  what  combination 
of  conductors  will  give  the  most  economical  results.  In 
making  any  comparison  between  the  copper  efficiencies  of 
the  various  systems,  the  proper  basis  of  comparison  is 
equality  of  voltage. 

The  amount  of  copper  required  for  transmitting  a  given 
power  at  a  fixed  percentage  loss  is  found  by  the  rule  that 
the  weight  of  copper  varies  inversely  as  the  square  of  the 
voltage. 

The  voltage  of  an  alternating  circuit,  as  measured  by 
the  ordinary  commercial  instruments,  —  i.e.,  the  effective 
voltage,  —  is  about  30  per  cent  less  than  the  maximum 
value  of  the  E.M.F.  wave.  It  is  this  maximum  value  that 
must  be  considered  in  determining  the  break-down  point 
of  insulation  and  the  highest  voltage  that  can  be  used 
commercially,  as  in  the  long-distance  transmission  of 
power.  On  the  other  hand,  when  the  maximum  voltage 
of  a  circuit  is  within  the  limit  of  safe  insulation  strain,  the 
effective  voltage  carries  no  limitation  with  it. 

The  comparison,  then,  of  the  various  systems,  to  deter- 
mine the  most  economical  method  of  transmission,  will  be 


336   POLYPHASE  APPARATUS  AND  SYSTEMS. 

either  on  the  basis  of  maximum  potential,  as  in  the  case  of 
long  transmission  lines,  or  on  the  basis  of  effective  or  min- 
imum potential,  as  in  the  case  of  low-potential  distributions 
by  secondary  mains. 

Figs.  199  to  204  show  the  standard  systems  of  alternat- 
ing-current distribution  and  the  various  combinations  of 
conductors  in  general  use.  The  name  of  each  system  is 
given,  and  also  the  relative  amount  of  copper  required. 

The  relative  amount  of  copper  required  by  the  single- 
phase  system,  which  is  here  taken  as  the  standard  of 
comparison  for  the  other  systems  and  combinations,  is 
illustated  by  diagram  (Fig.  199).  The  single-phase  three- 
wire  system  is  shown  in  Fig.  200.  If  the  voltage  of  the 
two-wire  system  is  e,  the  potential  between  the  two  outside 
wires  is  2e.  Applying  the  rule  that  the  amount  of  copper 
is  inversely  as  the  square  of  the  voltage,  only  J  the  copper 
would  be  needed,  if  the  neutral  should  have  no  cross  sec- 
tion, or  the  return  conductor  be  dispensed  with,  as  might 
be  done  in  the  case  of  a  perfect  balance.  If  the  neutral  is 
given  a  cross  section  equal  to  one  of  the  outside  wires,  the 
total  copper  in  the  three-wire  single-phase  system  is  37.5 
per  cent  that  of  the  two-wire  single-phase  system.  With 
a  neutral  -J-  and  ^  the  cross  section  of  the  outside  wires,  the 
total  copper  is  3 1.25  per  cent  and  29. 1 5  per  cent  respectively 
of  our  standard  system.  In  a  four-wire  single-phase  system 
the  voltage  between  outside  wires  is  3*,  and,  under  perfect 
balance,  |  the  amount  of  copper  would  be  required.  When 
the  neutral  and  outside  wires  are  of  equal  size,  the  copper 
must  be  increased  to  22.2  per  cent.  In  like  manner  the 
copper  in  the  five-wire  system,  with  neutrals  of  full  cross 
section,  is  1 5 .62  per  cent,  and  the  same  system,  with  neu- 
trals of  %  the  area  of  the  neutral  wires,  requiring  only  10.93 


RELATIVE    WEIGHTS    OF    COPPER. 


SYSTEM 


Single  Phase 
2  Wire 


Single  Phase 
8  Wire 


Two  Phase 
4  Wire 


Two  Phase 
3  Wire 


Three  Phase 
3  Wire 


Three  Phase 
4  Wire 


WIRING  CONNECTIONS 


Fig.  199. 


Fig.  200. 


Fig.  201. 


Fig.  202. 


Fig.  203. 


PER  CENT.' 
COPPER 

100. 


37.5 


100, 


73.9 


33.3 


337 

DIAGRAM 


I 
i 


L. 


A 


Fig.  204. 

per  cent  of  the  copper  of  the  simple  alternating  circuit. 
These  results  are  the  same  whether  the  comparison  is 
made  on  the  basis  of  maximum  potential,  or  on  the  basis 
of  effective  or  minimum  potential. 

In  the  three-phase  system,  the  copper  required  for  cer- 


338   POLYPHASE  APPARATUS  AND  SYSTEMS. 

tain  given  conditions  is  75  per  cent  of  the  copper  used  in 
the  single-phase  system.  The  comparison  between  poly- 
phase systems  can  best  be  made  by  resolving  each  into  as 
many  single-phase  systems  as  it  has  phases.  The  three- 
phase  system  consists  of  three  single  circuits  with  a  common 
ground,  or,  what  is  the  same,  with  no  return  ;  for  the  total 
current  to  and  from  the  centre  is  zero.  If  the  A  or  line  vol- 
tage is  e  (Fig.  203),  the  pressure  or  volts  between  any  wire 

/? 

and  the  juncture  is  — — .    The  single-phase  system,  having  a 

V3 
line  voltage  e,  can  also  be  converted  into  two  single  circuits 

g 
of  voltage  -  (Fig.  201).     As  the  weight  of  copper  in  each 

system  is  inversely  as  the  square  of  the  voltage,  we  have  : 

/  2  \2    /  V3  \2 

(-1:1  —  j  =4:3  —  or  the  relative  amounts  of  copper, 

for  the  single-phase  and  the  three-phase  systems,  are  100 
per  cent  and  75  per  cent.  Now  the  two-phase  four-wire 
system,  consisting  of  two  single-phase  systems,  is  placed, 
in  respect  to  the  amount  of  copper  required  for  equal  con- 
ditions, in  the  same  position  as  the  single-phase  system. 
Therefore  the  relative  amounts  of  copper  for  the  two-phase 
and  three-phase  systems  are  100  per  cent  and  75  per  cent. 
Fig.  202  illustrates  the  two-phase  three-wire  distribu- 
tion, two  of  the  wires  of  the  four-wire  system  being  replaced 
by  one  of  full  cross  section.  The  voltage  between  the 
two  outside  conductors  is  now  raised  to  e  V2  =  1.414  c,  e 
being  the  potential  between  the  conductors  of  either  phase. 
The  amount  of  copper  required,  when  compared  with  the 
single-phase  system,  will  differ  considerably  according  as 
the  comparison  is  based  on  the  highest  voltage  permissible 
for  any  given  distribution,  or  on  the  minimum  voltage  for 


RELATIVE    WEIGHTS    OF   COPPER.  339 

low-tension  service.      If  e  is  the  maximum   voltage   that 

can  be  used  on  account  of  the  insulation  strain,  or  for  any 

other  reason,  the  pressure  between  the  other  conductors 

of  the  two-phase  three-wire  system   must   be  reduced  to 

g 

—  „     The  weight  of  copper  required  under  this  condition 

V2 

is  145.7  per  cent  of  the  single-phase  copper.  If  the  limit- 
ing conditions  of  voltage  do  not  exist,  a  comparison  of  the 
relative  weights  of  copper  can  be  made  with  the  effective 
voltage  of  either  phase  as  a  basis,  —  i.e.,  on  a  basis  of  the 
minimum  voltage.  In  this  case  we  find  a  relative  saving 
over  the  single-phase  circuit  of  about  27  per  cent,  the 
actual  amount  of  copper  being  72.9  per  cent  of  the  single- 
phase  conductors. 

Fig.  204  shows  the  connections  of  the  three-phase  four- 
wire  system.  When  the  fourth  wire,  or  neutral,  is  of  full 
cross  section,  the  copper  required  is  331  per  cent  of  the 
single-phase  system.  By  making  the  neutral  one-half  the 
cross  section  of  the  main  conductors,  the  copper  weight  is 
reduced  to  29.17  per  cent.  This  arrangement  is  only  used 
for  secondary  systems  of  distribution,  as  described  before. 
The  comparison  with  any  other  system  is,  therefore,  made 
only  on  a  basis  of  equality  between  phases  of  minimum 
voltage. 

The  following  Tables  are  compiled  from  data  in  Mr. 
Steinmetz's  valuable  work,  "  Alternating-Current  Phenom- 
ena." The  first  Table  gives  the  relative  copper  efficiencies 
of  various  systems,  when  the  comparison  is  on  the  basis  of 
equality  of  minimum  difference  of  potential.  The  second 
gives  the  relative  weights,  when  the  comparison  is  based 
on  the  equality  of  the  maximum  potential  difference  in 
the  system. 


340 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


Amount  of  copper  required  for  transmission  at  a  given  loss,  based 
on  minimum  potential. 


SYSTEM. 

No.  OF  WIRES. 

PER  CENT 

COPPER. 

Single-phase    .                       

2 

IOO 

Single-phase                                . 

•5 

•77  r 

J/-J 

Two-phase,  common  return  .     •     .     .     . 

3 

72.9 

Two-phase   '. 

4 

IOO 

7 

7C 

Three-phase,  neutral  full  section  .     .     . 

4 

33-3 

Three-phase,  neutral  one-half  section    . 

4 

29.17 

Amount  of  copper  required  for  transmission  at  a  given  loss,  based 
on  maximum  difference  of  potential. 


SYSTEM. 

No.  OF  WIRES. 

PER  CENT 
COPPER. 

Single-phase     

2 

IOO. 

Two-phase,  with  common  return  .     .     . 

3 
4 

145-7 
IOO. 

Three-phase          

•j 

7C. 

Direct  Current               .              .... 

2 

so. 

It  will  be  seen  that  the  direct-current  system  requires 
only  50  per  cent  of  the  copper  in  the  single-phase  system 
when  used  in  long-distance  transmission  of  power.  The 
advantage  is  not  so  evident,  however  ;  for,  as  Mr.  Stein- 
metz  has  pointed  out,  in  addition  to  the  electrostatic  stress, 
an  electrolytic  effect  is  set  up,  which  does  not'  exist  to  the 
same  extent  in  alternating  currents.  The  complications 
attending  the  utilization  of  direct  current  of  high  tension, 
are  such  that,  with  the  exception  of  a  few  special  cases,  its 
employment  in  the  long-distance  transmission  of  power  has 
not  been  considered  practicable. 


CALCULATION    OF    TRANSMISSION    LINES.  34! 


CHAPTER  XVI. 
CALCULATION   OF  TRANSMISSION    LINES. 

Line  Constants. — As  explained  in  Chapter  I.,  the  drop 
of  voltage  in  an  alternating-current  circuit  will  vary  with 
the  resistance  and  the  reactance  of  the  circuit,  and  with 
the  character  of  the  load.  In  the  table,  "  Line  Constants 
for  Power  Transmission,"  taken  from  a  publication  of  the 
General  Electric  Company,  the  relation  of  reactance  to 
resistance  is  shown  for  a  number  of  frequencies,  and  for 
the  sizes  of  conductors  ordinarily  used  in  power  transmis- 
sions, and  also  other  constants  of  transmission  circuits, 
such  as  capacity,  inductance,  and  charging  current.  The 
following  explanations  will  serve  to  make  the  table  clear : 

The  E.M.F.  consumed  by  resistance  r,  of  the  line,  is  =  Ir, 
and  in  phase  with  the  current  / 

The  E.M.F.  consumed  by  the  reactance,  S,  of  the  line,  is  = 
73",  and  in  quadrature  with  the  current  7. 

The  E.M.F.  consumed  in  the  line  is  neither  Ir  nor  IS,  but 
depends  upon  the  phase  relation  of  current  in  the  receiving 
circuit. 

The  loss  of  energy  in  the  line  is  =  /V,  hence  does  not  de- 
pend upon  the  reactance,  but  only  upon  the  resistance. 

Two  wires  in  parallel  have  the  same  resistance,  and  about 
half  the  reactance  (if  strung  on  separate  insulators  and  inter- 
mixed) of  a  single  wire  of  double  cross  section.  Thus  replacing 
one  No.  oooo  wire  by  two  No.  o  wires,  the  resistance,  weight 


342     POLYPHASE  APPARATUS  AND  SYSTEMS. 

of  copper,  etc.,  will  remain  the  same,  but  the  reactance  will  be 
reduced  practically  to  half,  so  where  lower  reactance  is  desired, 
the  use  of  several  conductors  strung  on  independent  insulators 
and  intermixed  is  advisable. 

The  values  given  for  Z,  C,  i  ,  and  S  are  calculated  for  sine- 
waves  of  current  and  E.M.F. 

This  table  will  be  found  most  convenient  for  determin- 
ing the  characteristics  of  transmission  circuits  when  the 
size  of  conductor  has  been  fixed.  The  conductors  are  as- 
sumed to  be  separated  by  a  distance  of  18  inches. 

Let  us  take,  as  an  example,  a  case  where  it  is  required 
to  deliver,  by  the  three-phase  60  cycle  system,  2,000  H.P. 
at  the  secondary  terminals  of  the  step-down  transformers, 
over  a  circuit  1  1  miles  in  length.  It  is  further  assumed 
that  the  voltage  at  the  receiving  end  is  10,000,  and  the 
total  energy  loss  in  transmission  from  the  generator  ter- 
minals is  not  to  exceed  10  per  cent.  The  power  is  to  be 
used  for  a  mixed  system  of  lights  and  induction  motors, 
the  latter  forming  most  of  the  load.  The  power  factor  of 
the  system  at  the  receiving  end  will  be  approximately  85 
per  cent.  We  can  assume  that  — 

The  transformers  have  an  efficiency  of  97^-  per  cent. 
The  copper  loss  in  each  being  i  per  cent. 
The  core  or  hysteresis  loss,  i^-  per  cent. 
The  reactance  can  be  taken  as  3^-  per  cent. 
And  the  magnetizing  current  4  per  cent. 
The  voltage  between  any  branch  of  the  circuit  and  the 
common  centre  of  the  system  is 


=  5,775. 
V3 

The  energy  delivered  by  each  branch  is 

K.W. 


CALCULATION   OF   TRANSMISSION    LINES.          343 

The  apparent  energy  delivered  by  each  branch  is 
-^  -  588  K.W. 

The  total  current  in  each  branch  is =  102 

amperes. 

The  LR.  drop  in  each  branch  is  10  per  cent  of  5,775  = 
577-5  volts. 

The  total  resistance  R  =  -     -  =  5.66  ohms. 

102 

5.66 
The  resistance  of  one  mile  is  -    -  =  o.  5 1 4  ohm,  which 

is  very  nearly  the  resistance  of  No.  o  wire.  Three  No.  o 
wires,  therefore,  will  carry  2,000  H.P.  a  distance' of  n 
miles  with  a  waste  of  energy  of  10  per  cent,  the  pressure 
at  the  receiving  end  being  10,000  volts  and  power  factor 
85  per  cent. 

By  referring  to  the  table  the  characteristics  of  this 
transmission  line  are  readily  obtained.  The  reactance  of 
eleven  miles  of  single  conductor  is  seen  to  be  6.62  ohms 
at  the  frequency  employed.  The  inductance,  or  what  is 
the  same  thing,  the  coefficient  of  self-induction  of  the  line, 
is  17.6  millihenrys.  The  charging  current  of  each  line  for 
the  eleven  miles,  with  the  given  voltage  and  frequency,  is 
found  to  be  0.4  ampere. 

It  is  interesting  to  know  what  the  impressed  or  genera- 
tor E.M.F.  and  the  distribution  of  current  will  be,  in  this 
case,  when  the  plant  is  fully  loaded.  For  this  investiga- 
tion, the  entire  system  may  be  reduced  to  a  uniform  vol- 
tage, by  multiplying  the  voltages  by  the  various  ratios  of 
transformation,  thus  bringing  both  the  secondary  pressure 
at  the  step-down  transformers,  and  the  generator  pressure, 
to  the  line  voltage.  The  current  values  are,  of  course, 
inversely  changed.  The  power  factor  of  the  load,  hav- 
ing been  assumed  as  0.85,  the  induction  factor  will  be 
Vi-(o.85)2  =0.52. 


344 


POLYPHASE  APPARATUS  AND  SYSTEMS 


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CALCULATION    OF   TRANSMISSION    LINES, 


345 


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346 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


In  Chapter  I.,  it  has  been  shown  that  the  impressed 
E.M.F.  is  made  up  of  two  component  parts,  one  in  phase 
with  the  current  and  called  the  energy  component  of  the 
E.M.F.,  the  other  in  quadrature  with  the  current  and 
called  the  induction  component.  In  symbols  : 

Impressed  E.M.F. 


=  V2  (Energy  comp.)2  -f-  2  (Ind.  comp.)2 

To  obtain  the  total  E.M.F.  it  is  necessary,  then,  to  calcu- 
late separately  all  the  energy  and  induction  components  of 
the  circuit,  and  obtain  a  combined  resultant. 

With  the  values  already  assumed,  and  consulting  the 
preceding  table,  we  obtain  the  following  results  : 


CIRCUIT. 

VOLTAGE. 

CURRENT 
AMPERES. 

ENERGY 
COMPO- 
NENT. 

IND.  COM- 
PONENT. 

Secondary  Circuit. 

Energy  Component,      .85  X  5,775, 
Induction  Component,  .52  X  5,775, 

4,909 

3,°°3 

Current, 

102 

Step-down  Transformers. 

Resistance  loss  =  LR.  =  i%  of  5,775, 

58 

Reactance  loss  =  S.S.  =  3^%  of  5,775, 
Hysteresis  loss  =  i%%  of  102, 

2O2 

i-5 

4,967 

3,2°5 

103.5 

Line. 

Resistance  loss  =  LR.  =  103.5  X  5.72, 

592 

Reactance  loss  =  7.5".  =  103.5  x  6.62, 

685 

J  (5,559)2  +  (3,890)*  =  6,785=  volts  at 
terminals  of  step-up  transformers. 

5,559 

3,890 

I03-5 

Step-up  Transformers. 

Resistance  loss  =  LR.  =  i%  of  6,785, 

68 

Reactance  loss  =  LS.  =  3^%  of  6,785, 

238 

Hysteresis  loss               =  i)|%of  103.5, 

'•5 

+/  (5,628)2  +  (4,128)*  =  6,980  =  volts  at 

generator. 

,5,627 

4,128 

105. 

CALCULATION    OF   TRANSMISSION    LINES.  347 

The  energy  E.M.F.  between  any  one  line  and  the  neutral 
at  the  generator  end  is  seen  to  be  5,627,  and  the  volts  con- 
sumed by  the  reactance  of  the  system,  4,128.  The  total 
volts  required  at  the  generator  terminals  are  found  to  be 
1 2 1  per  cent  of  the  voltage  at  the  secondaries  of  the  trans- 
formers, reduced  to  the  line  voltage,  —  i.e.,  with  10,000 
equivalent  volts  between  the  lines  at  the  transformer  sec- 
ondaries, the  pressure  at  the  generator  must  be  12,100 
volts.  The  current  delivered  by  the  generator  to  the  line 
is  105  amperes,  and  is  3  per  cent  more  than  the  current  in 
the  secondary  circuits.  The  effect  of  the  transformer  core 
losses  is  the  same  as  if  a  corresponding  current  was  con- 
sumed by  lamps  or  other  apparatus  connected  across  the 
mains.  The  volt-ampere  output  of  the  generator  is  125  per 
cent  of  the  apparent  watts  at  the  receiving  end.  The  power 
factor  of  the  entire  system  is  found  to  be  about  So  per  cent. 

Simple  Wiring  Formulas. —  A  simple  and  sufficiently 
accurate  determination  of  the  sizes  of  conductors,  voltage 
drop,  and  distribution  of  currents,  in  any  direct  or  alternat- 
ing-current system,  can  be  made  from  the  general  formula 
based  on  Ohm's  law,  modified  by  the  use  of  the  proper 
constants.  The  former  formula  and  constants  will  be  found 
especially  useful  and  convenient  for  this  calculation : 

D  x  W 

Area  of  conductor,  Circular  Mils  =  — —  x  K 

P  X  E 

Volts  loss  in  lines  = X  M 

100 

W 

Current  in  main  conductors  =  —  X  T 

XL* 

D  =  Distance  of  transmission  (one  way)  in  feet. 
W  =  Total  watts  delivered  to  consumer. 
P  =  Per  cent  loss  in  line  of  W. 

E  =  Voltage  between  main  conductors  at  receiving  or  con- 
sumer's end  of  circuit. 


348 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


VALUES  OF  K. 

VALUES  ol  T. 

SYSTEM. 

PER    CENT    POWER    FACTOR. 

PER   CENT    POWER    FACTOR? 

IOO 

95 

90 

85 

80 

95 

90 

85 

80 

Single-phase  . 

2,  1  60 

2,400 

2,660 

3,000 

3,38o 

1.052 

I.  Ill 

1.176 

1.250 

Two-phase  (4- 

wire)  .    .     . 

1,  080 

I,2OO 

1,33° 

1,500 

1,690 

.526 

•555 

.588 

.625 

Three  -  phase 

(3-wire)   .     . 

1,  080 

I,2OO 

!,33o 

1,500 

1,690 

.607 

.642 

.679 

•725 

Values  of  the  constant,  K,  for  any  particular  power  factor 
are  obtained  by  dividing  2,160  by  the  square  of  that  power 
factor  for  single-phase,  and  by  twice  the  square  of  that 
power  factor  for  three-wire  three-phase  or  four-wire  two- 
phase.  The  resistance  of  line  wire  is  taken  as  10.8  ohms 
per  mil  foot. 

T  is  a  variable,  depending  on  the  system  and  nature  of 
the  load,  and  equal  to  i  for  continuous  current,  and  for 
alternating  current  with  100  per  cent  power  factor.  Its 
value  for  two-phase  and  three-phase  systems  is  0.50  and 
0.58  respectively,  with  100  per  cent  power  factor. 

M  is  a  variable,  depending  on  the  size  of  wire,  frequency, 
and  power  factor.  It  is  equal  to  i  for  continuous  cur- 
rent, and  for  alternating  current  with  100  per  cent  power 
factor  and  sizes  of  wire  given  in  the  following  table  of 
wiring  constants. 

The  values  of  M,  as  given  in  the  table,  are  empirical. 
They  are  sufficiently  accurate  for  all  practical  purposes, 
provided  the  displacement  in  phase  between  current  and 
E.  M.  F.  at  the  receiving  end  is  not  very  much  greater  than 
that  at  the  generator  ;  in  other  words,  provided  that  react- 
ance of  the  line  is  not  excessively  large,  or  the  line  loss 
unusually  high.  For  example,  the  constants  should  not  be 


CALCULATION    OF   TRANSMISSION    LINES.  349 

applied  at  125  cycles  if  the  largest-size  conductors  were 
used,  and  the  loss  20  per  cent  or  more  of  the  power  deliv- 
ered. At  lower  frequencies,  however,  the  constants  are 
reasonably  correct,  even  under  such  extreme  conditions. 
They  represent  about  the  true  values  at  10  per  cent  line 
loss,  are  close  enough  at  all  losses  less  than  10  per  cent, 
and  often,  at  least  for  frequencies  up  to  40  cycles,  close 
enough  for  even  much  larger  losses. 

In  using  the  above  formulas  and  constants,  it  should  be 
particularly  observed  that  P  stands  for  the  per  cent  loss  in 
the  line  of  the  delivered  power,  and  not  for  the  per  cent 
loss  in  line  of  the  power  at  the  generator. 


VALUES   OF  M. 

No. 

WEIGHT 
OF  BARE 

30  CYCLES. 

60  CYCLES. 

125  CYCLES. 

OF 

AREA 

WIRE 

WIRE 

CIRCULAR 

PER 

• 

. 

B.  & 

MILS. 

^       Cti 

^  H    • 

1/1     &J 

o      • 

'•A         b". 

o      • 

V}    ^      ' 

en 

S.  G. 

POUNDS. 

K  ^ 
oO^> 

°  «  P-! 

sgl 

Ml 

H  jPW 

If 

Jlj 

3         £ 

0   ^CL 

IgS 

II 

oooo 

2II,6oo 

640.73 

1.26 

1.27 

1.24 

1.64 

1.85 

1.85 

2-44 

3.06 

3.14 

000 

167,805 

508.12 

1.  2O 

1.17 

1.14 

1.49 

1.63 

1.62 

2.15 

2.62 

2.67 

oo 

133,079 

402.97 

I.I5 

i.  08 

1.05 

'•39 

1.46 

1.42 

1.92 

2.25 

2.29 

0 

105,592 

3*9-74 

1.  10 

I.OO 

I.OO 

1.30 

1.32 

1.28 

1-73 

1.96 

1.99 

I 

83,694 

253-43 

1.  06 

I.OO 

I.OO 

1.23 

1.  21 

1.16 

L57 

1.74 

1-73 

2 

66,373 

200.98 

1.03 

I.OO 

I.OO 

1.16 

I.  II 

1.  06 

1.44 

1.54 

i-53 

3 

52,633 

159.38 

1.02 

I.OO 

I.OO 

i.  ii 

1.04 

I.OO 

i-35 

1.38 

1.38 

4 

41,742 

126.40 

I.OO 

I.OO 

I.OO 

1.07 

I.OO 

I.OO 

1.26 

1.26 

1.22 

5 

33>102 

100.23 

I.OO 

I.OO 

I.OO 

1.04 

I.OO 

I.OO 

1.19 

1.16 

I.  II 

6 

26,250 

79-49 

I.OO 

I.OO 

I.OO 

1.02 

I.OO 

I.OO 

1.14 

i.  08 

1.03 

7 

20,816 

63-03 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

1.09 

I.OI 

I.OO 

8 

16,509 

49.99 

I.  CO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

i.  06 

I.OO 

I.OO 

350  POLYPHASE   APPARATUS    AND    SYSTEMS. 

APPLICATION  OP  FORMULAS. 
SINGLE-PHASE    SYSTEM.  -  12$    CYCLES. 

EXAMPLE:  750  52-volt  lamps,  consuming  a  total  of 
45,000  watts.  Ratio  of  transformation  20  to  i.  Distance 
to  generator,  2,500  feet.  Loss  in  secondary  wiring,  2  volts. 
Voltage  drop  in  transformers,  2  per  cent.  Energy  loss  in 
line,  5  per  cent  of  delivered  power.  Efficiency  of  trans- 
formers, 97£  per  cent. 


Watts  at  transformer  primaries 

4C.OOO 

=  -  —  -  =  47,100. 
0.98  X  0.97^ 

Volts  at  transformer  primaries 

=  (52  +  2)  X  20  x  1.02  =  1,101.6. 
r>  as       D*W  2,500X47,100X2,400 

CJf'-T*0**=  5  x  (.,101.6)*          =  46,500  C.J/: 

Next  larger  B.  &  S.  wire 

=  No.  3  =  52,633  C.M. 
Loss  of  delivered  power  using  No.  4  wire 

2,500  X  47,100  X  2,400 


—  —  z  —      /  * 

52,633  x  (i,ioi.6)2 

Total  volts  lost  in  line 


=  4-4  per  cent. 


x  jy=        x  T>101'6  x  T-35  =  6 


100  100 

Generator  voltage        =  1,101.6  +  65.5  =  1,167.1. 

In  a  60  cycle  single-phase  system,  with  the  same  condi- 
tions as  in  the  above  example,  the  values  will  be  the  same, 
with  the  exception  of  the  volts  lost  in  the  line. 

4.4  x  1,101.6  x  i.  ii 

—  C3.8  =  volts  lost  in  line. 


100 
1,101.6  +  53.8  =  1,155.4  =  generator  voltage. 


CALCULATION    OF   TRANSMISSION    LINES.  351 

TWO-PHASE    SYSTEM.  -  60    CYCLES.       FOUR-WIRE 
TRANSMISSION. 

EXAMPLE  :  2,500  H.  P.  delivered,  5  miles,  at  secondaries 
of  step-down  transformers.  Pressure  between  lines  at  re- 
ceiving end,  6,000  volts.  Energy  loss  in  line  and  in  step- 
down  transformers  (no  step-up  transformers),  10  per  cent 
of  delivered  power.  Efficiency  of  transformers,  97.5  per 
cent.  Power  factor  of  load,  80  per  cent.  Find  size  of 
conductors  and  voltage  drop  in  transmission  line. 
Power  delivered  at  step-down  primaries. 

HP  =         2  ;  KWt 


°-975 

Energy  loss  in  line  =  7.5  per  cent. 
5,280  X  z  X  1,012,700 
C'M-  =  -    7.5  x  (6,ooo/  - 
Three  No.  o  B.  &  S.  wires  have  an  area  of  316,776  C.M. 
The  energy  loss,  using  3  of  this  size  in  parallel,  making  a 
total  of  12  No.  o  B.  &  S.  wires  in  all,  is  : 

5,280  x  5  x  1,912,700 

—  -  1  -  j^-  —  ^5-  x  1,690  =  7.48  per  cent. 
316,776  x  (6,ooo)2 

Power  lost  in  line 

=  2,564  X  0.0748  =  195.8  H.P. 
Volts  lost  in  line 

P  X  E  7.48  X  6,000  X  1.28 

=  -       -xJf=—  -  =  574- 

100  100 

.*.  Generator  voltage  =  6,574. 

Current  in  line 

W  1,012,700 

=  —  x  T=  —^  —  -  —  X  .625  =  199  amperes. 

XL  O,OOO 

The  current  is,  in  fact,  slightly  greater,  as  no  account 
has  been  taken  of  the  hysteresis  current  in  the  trans- 
formers. This  will  increase  the  above  result  about  i-J  per 
cent. 


352  POLYPHASE   APPARATUS   AND    SYSTEMS. 

THREE-PHASE    SYSTEM.  -  6O    CYCLES.       THREE-WIRE 
TRANSMISSION. 

EXAMPLE  :  Same  conditions  as  preceding.     Find  size  of 
conductors  and  voltage  drop  in  transmission  lines. 

Power  delivered  to  transformers 

=  =  2.5<HH.P.  =  1,912.7  K.W. 


Energy  loss  in  line  =  7^-  per  cent. 

5,280  x  5  x  1,912,700 

C.M.  =  -  -2-^  —  X  1,690  =  315,940  C.M. 

7.5  x  (6,000)^ 

Three  No.  o  B.  &  S.  wires  have  an  area  of  316,776  C.M. 
For  the  three  branches  of  the  three-phase  system  9  wires 
will  be  required. 

T7  i       •         5,280  X  5  X  1,912,700 

Energy  loss  is  =  -  —  -  -  §  -  7-?  —  '4-*-  X  1,690  =  7.48  percent. 
316,776  X  (6,ooo)2 

Power  loss  in  line 

=  2,564  X  0.0748  =  195.8  H.P. 
Voltage  drop  in  line 

_  7.48  x  6,000  X  1.28 


.-.  Generator  voltage  =  6,574. 
Current  in  line 

1,912,700 
6?000      *  °-725  =  233-9  amperes. 

The  hysteresis  current  will  increase  this  result  by  about 
li  per  cent. 

THREE-PHASE    SYSTEM.  -  60    CYCLES.       FOUR-WIRE 
SECONDARY. 

EXAMPLE  :  Required,  the  size  of  conductors  from  trans- 
formers to  the  distributing  centre  of  a  four-wire  secondary 
system  for  lights  and  motors.  The  load  consists  of  four 


CALCULATION    OF   TRANSMISSION    LINES.  353 

15  H.P.,  200  volt  induction  motors,  and  750  half -ampere 
1 5  c.p.,  1 1 5  volt  lamps.  Length  of  secondary  wiring  from 
transformers  to  distribution  centre,  600  feet.  About  15 
volts  drop  on  lighting  circuits  from  transformers  to  distrib- 
uting centre.  Efficiency  of  motors,  85  per  cent.  Five 
volts  drop  on  circuits  from  distributing  centre  to  motors. 
Voltage  at  distributing  point  between  main  lines  is  205 
Current  in  main  lines  for  motors  is 

4  X  it;  X  746  X  o.72t; 

*  Q    '  —L^-  =191  amperes. 

0.85    X    200 

Current  from  transformers  for  lamps  is 
(750  x  0.5  x  115)  x  0.607 

-  =  131  amperes. 
200 

Total  current  from  transformers  is 

131-1-191—322  amperes. 
For  motors, 

W 
191  = x  0.725.     W=  54,000. 

For  lamps, 

W 
131  =  —  -  X  0.607.    W '=  44,240.     Total  watts  =  98,240. 

205 

Taking  for  trial  two  No.  o  B.  &  S.  wires  in  parallel  for 
each  of  the  main  conductors  as  preferable  to  one  No.  oooo, 
then 

600  X  98,240 
~  2  X  105,592   X  205* 
1,200  X  44,240  4-  1,690  X  54,000  _ 

98,240 
Volts  loss  in  lines 

=  9-75  X  205  x  1.32  = 

100 

Volts  at  transformers  between  main  lines  =  231.4. 
Actual  drop  between  main  conductors  and  neutral  to  distrib- 
uting point 

=  26.4  X  — -  =  15.2  volts. 

200  3 


354     POLYPHASE  APPARATUS  AND  SYSTEMS. 


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CALCULATION    OF   TRANSMISSION    LINES.  355 


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POLYPHASE  APPARATUS  AND  SYSTEMS. 


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CALCULATION   OF  TRANSMISSION    LINES.  357 

The  section  of  the  neutral  conductor  should  be  about 
131  X  2  X  I05>592  =  86  CM  We  use  Qne  NQ  l 

322 
B.  &  S.  wire  with  a  section  of  83,694  C.M.  for  the  neutral. 

Graphical  Illustration.  —  The  curves  on  pages  354-356, 
Figs.  205,  206,  and  207,  have  been  calculated  from  the 
preceding  formula  and  table  of  constants  relating  to  the 
three-phase  system  only.  They  will  be  found  useful  for 
calculating  and  approximately  determining  the  copper  re- 
quired for  transmitting  any  amount  of  power  any  distance 
at  voltages  varying  from  1,000  to  15,000. 

For  cases  that  fall  outside  the  limits  of  the  curves,  the 
size  of  the  wire  may  be  found  by  applying  the  following 
rules  : 

With  given  power  delivered,  line  loss,  and  voltage,  the  cross 
section  of  the  conductor  will  vary  directly  as  the  distance. 

With  given  distance  of  transmission,  line  loss,  and  voltage, 
the  cross  section  of  the  conductor  will  vary  directly  as  the  power 
delivered. 

With  given  distance  of  transmission,  power  delivered,  and 
voltage,  the  cross  section  of  the  conductor  will  vary  inversely 
as  the  loss  of  energy  in  the  line. 

With  given  distance,  power  delivered,  and  line  loss,  the  cross 
section  of  the  conductor  will  vary  inversely  as  the  square  of 
the  voltage. 

The  voltages  are  taken  as  those  at  the  receiving  end. 
The  line  loss  has  been  assumed  to  be  10  per  cent  of  the 
delivered  energy.  In  plotting  the  curves  the  following 
power  factors  have  been  assumed  : 

For  lighting  load 95% 

For  mixed  load  of  induction  motors  and  lights    .     .     85  % 
For  induction  motor  load 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


To  illustrate  the  use  of  the  curves,  find  the  size  of  the 
wire  required  to  transmit  5,100  H.P.,  to  be  used  for  incan- 
descent lighting,  a  distance  of  five  miles,  the  current  loss 
being  10  per  cent,  and  the  pressure  at  the  primaries  of 
step-down  transformers,  10,000  volts.  The  curve  (Fig. 
167)  shows  that  each  of  the  three  wires  must  have  a  cross 
section  of  1  20,000  circular  mils.  If  the  power  delivered  is 
to  be  consumed  by  induction  motors,  other  conditions  re- 
maining the  same,  the  conductor  must  have  a  cross  section 
equivalent  to  170,000  circular  mils  each,  or  slightly  larger 
than  No.  ooo  wire.  Or,  supposing  the  wire  to  have  been 
strung  on  the  assumption  that  lights  would  be  supplied, 
the  line  loss  and  pressure  being  the  same  as  above,  it  will 
be  seen  that,  if  the  load  is  changed  to  induction  motors, 
only  3,600  H.P.  will  be  delivered  from  these  lines.  This  is  a 
striking  illustration  of  the  decrease  in  the  carrying  capacity 
of  the  line,  due  to  low  power  factors,  which  load  the  line, 
and  the  generators  as  well,  with  so-called  wattless  current. 

If  the  distance  is  raised  to  ten  miles,  the  size  of  wire 
required  for  the  same  transmission  is  doubled  in  both  the 
above  examples.  If  the  distance  is  increased  to  ten  miles, 
and  the  energy  loss  reduced  to  five  per  cent,  the  cross 
section  of  conductor  will  have  to  be  made  four  times  as 
great. 

Three  wires  of  about  No.  3  size  will  transmit  a  lighting 
load  of  5,100  H.P.  a  distance  of  five  miles,  the  pressure 
being  15,000  at  the  receiving  end.  It  will  take  three  con- 
ductors of  cross  section  corresponding  to  a  size  between 
No.  i  and  No.  2  to  transmit  the  same  power  for  induction 
motor  use,  and  three  No.  2  wires  to  transmit  the  same 
energy  for  a  mixed  load  of  lights  and  motors. 

For  determining  the  size  of  transmission  lines  with  vol- 


CALCULATION   OF   TRANSMISSION    LINES. 


tages  of  5,000  and  less,  the  curves  in  Fig.  206  will  be 
found  most  convenient. 

Fig.  207  represents  the  curves  of  percentage  drop  of 
voltage  in  transmission  lines,  at  varying  frequencies  and 
power  factors.  The  curves  show  the  values  of  the  con- 
stant, M,  plotted  from  the  table  on  page  349,  and  are  based 
on  10  per  cent  energy  loss  in  line. 

A  study  of  the  curves  shows  some  interesting  facts. 
When  the  transmission  is  effected  at  30  cycles,  it  will  be 
noticed  that,  for  all  commercial  sizes  of  wires,  the  voltage 
drop  is  less  with  a  load  of  low  power  factor  than  with 
one  of  high  power  factor.  For  illustration,  assume  that 
the  transmission  requires  conductors  of  120,000  circular 
mils  each,  the  energy  loss  being  10  per  cent  of  the  deliv- 
ered power.  At  30  cycles,  the  drop  in  voltage  is  10.1 
per  cent  when  the  power  factor  is  80  per  cent,  10.4  per 
cent  when  the  power  factor  is  85  per  cent,  and  11.3  per 
cent  when  the  power  factor  is  95  per  cent.  On  the  other 
hand,  the  same  transmission  at  125  cycles  shows  a  higher 
voltage  drop  with  low  power  factors.  The  voltage  drops 
18.3  per  cent  with  a  95  per  cent  power  factor,  21.2  per 
cent  with  an  85  per  cent  power  factor,  and  21.7  per  cent 
when  the  power  factor  is  80  per  cent. 

A  curious  condition  exists  at  60  cycles.  The  voltage 
drop  is  less  with  a  power  factor  of  95  per  cent,  than  when 
the  power  factor  is  85  per  cent  ;  but  an  80  per  cent 
power  factor  gives  a  drop  approximately  the  same  as  that 
due  to  a  power  factor  of  95  per  cent.  The  curves  also 
graphically  illustrate  the  reduction  in  voltage  drop  to  be 
gained  by  subdividing  the  conductors.  A  No.  oo  wire, 
used  in  a  60  cycle  transmission  of  power  for  induction 
motors,  shows  a  drop  of  14.3  per  cent.  By  subdividing 
the  wire  into  two  No.  2  wires,  and  equivalent  cross  sec- 
tion, the  voltage  drop  is  reduced  to  10.6  per  cent. 


360     POLYPHASE  APPARATUS  AND  SYSTEMS. 

It  will  bs  seen,  from  the  curves,  that,  by  subdividing 
the  conductor  sufficiently,  a  wire  of  a  size  can  be  selected, 
which,  for  all  commercial  power  factors  and  frequencies, 
will  transmit  any  amount  of  power,  with  a  drop  of  voltage 
in  the  line  actually  less  than  the  energy  loss.  This  ap- 
parent anomaly  is  explained  in  Chapter  I.,  under  the  para- 
graph/'Voltage  Drop  Dependent  on  Load  Characteristic." 

Resonance  Effect What  is  known  as  the  resonance 

effect  of  a  circuit  is  the  rise  of  E.M.F,  at  the  far  end,  above 
that  at  the  generator  end.  This  phenomenon  takes  place 
when  the  natural  period  of  discharge  of  a  circuit  is  equal 
to  the  frequency  of  the  generator  E.M.F.  It  is  complete 
when  the  self-induction  and  capacity  exactly  neutralize 
each  other.  The  charging  current  of  the  line,  due  to  the 
capacity,  then  produces  an  E.M.F.  of  self-induction  equal 
to  the  generator  E.M.F. 

In  transmission  lines,  where  the  inductance  and  capacity 
do  not  exactly  neutralize  each  other,  it  is  possible  for  par- 
tial resonance  to  be  present.  The  circuit  can  be  brought 
into  complete  resonance  by  the  addition  of  a  condenser  or 
a  reactance,  according  as  it  lacks  the  proper  amount  of 
either  capacity  or  inductance.  It  is  conceivable  that  an 
unexpected  rise  of  pressure  may  occur  of  sufficient  extent 
to  destroy  the  insulation  of  line  and  of  apparatus. 

The  rise  of  pressure  due  to  complete  resonance  is  limited 
by  the  ohmic  resistance  of  the  circuit.  For  this  reason, 
and  because  practical  transmissions  of  power  are  accom- 
plished at  a  comparatively  low  frequency,  the  possible  rise 
of  pressure  at  the  receiving  end  is  not  likely  to  be  danger- 
ously high. 

For  very  long  power  transmissions,  where  resonance 
effects  may  be  expected,  it  is  desirable  to  employ  genera- 
tors producing  an  E.M.F.  wave  which  is  sinusoidal.  A 


CALCULATION    OF   TRANSMISSION    LINES.          361 

distorted  wave  of  E.M.F.  of  the  same  period  can  be  resolved 
into  a  number  of  simple  harmonic  components  of  a  higher 
frequency.  These  higher  harmonics  have  the  same  effect 
as  an  E.M.F.  wave  of  the  same  frequency  and  magnitude. 
Another  form  of  resonance  effect  is  that  which  occurs 
during  both  the  making  and  the  disruption  of  a  current  of 
high  tension,  especially  in  long  distance  transmission  lines. 
It  is  found  that  the  oscillatory  discharge  in  the  arc  under 
these  conditions  may  become  of  such  magnitude  as  to  seri- 
ously endanger  the  insulation  of  the  line  and  of  apparatus 
connected  thereto.  As  has  been  described  in  the  section 
under  high  tension  switches,  this  effect  is  particularly 
noticeable  in  those  interruptions  to  the  current  which  are 
effected  in  the  open  air. 


INDEX. 


Air  blast  transformers,  173. 
Alternating  circuit,  energy  in,  14. 

flow  of  current  in,  7. 
Alternations,  definition,  20. 
Alternators  (see  Generators). 
Amortisscur    winding    for    synchro- 
nous motors,  146, 
Amplitude,  6. 
Apparent  efficiency,  115. 
Apparent  energy,  14. 
Apparent  resistance,  14. 
Arc    lamps   on    low   frequency    cir- 
cuits, 331. 

Armature  reaction,  42,  43. 
Armature  inductance,  41. 
Armature,  induction  motors,  84. 

multitooth  construction,  40. 

reaction  of  generators,  39,  40. 
Armature,   resistance    of    induction 
motors,  85. 

unitooth  construction,  39. 
Auto   transformers   (see   Compensa- 
tor). 

Balanced  three-phase  system,  318. 

two-phase  system,  302,  307. 
Blowers,    for    cooling   transformers, 

I76. 
Break -down  point,  induction  motors, 

98,  102. 

synchronous  motors,  134. 
Bridges,  26. 


Capacity,  8. 

Capacity  and  magnetic  reactance  in 

same  circuit,  12. 

Capacity  of  transmission  lines,  344. 
Capacity,  unit  of,  9. 
Charging    current    in    transmission 

lines,  344. 
Choke  coil,  288. 
Circuit  breakers  (see  Switches). 
Compensators,  for  induction  motors, 

86,  87,  88. 
Compensators,       for      synchronous 

motors,  139. 

Compensator,  line  drop,  280. 
Compounding 'of  generators,  45. 
.  Compound  winding  for  generators, 

46. 

Condcnsance,  12. 
Condenser,    use   of,    with   induction 

motors,  117. 
Condenser,  synchronous  motor  used 

as,  150. 

Conductors  (see  Transmission  lines). 
Control  of  alternating-current  appa- 
ratus, 244. 
Converter  (see  Rotary  converter  and 

Motor  converter). 
Converting  apparatus,  miscellaneous, 

224. 
Copper,    amount  of,    required   with 

different  systems,  335. 
Cosine  of  lag  angle,  15. 


363 


3^4 


INDEX. 


Currents,   alternating,   definition  of 

terms,  i. 

Current,    armature,    in    rotary   con- 
verter, 205. 

in  synchronous  motor,  140. 
Current,  lagging,  8. 
leading,  q. 
wattless,  1 6. 

Delta  connection  of  windings,  300, 

3i2>  3i3- 
Distribution     circuits,     three-phase, 

four-wire,  314,  317. 
three-phase  three-wire,  312,  316. 
two-phase  four-wire,  304. 
two-phase  three-wire,  307. 
Dynamometer  effect   of   alternating 
current,  4. 

Efficiency,  generators,  53. 

induction  motors,  98,  115,  118. 

synchronous  motors,  133. 

transformers,  180. 

Electromotive  force  (see  also  Volt- 
age). 
Electromotive  force,  2. 

curve  of,  2,  6. 

curve  of,  three-phase,  311. 

curve  of,  two-phase,  299. 

drop    in,    of    alternators     due    to 
load,  41. 

energy  component  of,  10. 

induction  component  of,  10. 

impressed,  9» 

sinusoidal,  2. 

Energy,  calculation  of,  in  alternat- 
ing-current circuits,  14, 
Energy,  apparent,  14. 

current,  16. 

loss  in  circuit,  17. 


Engine,    regulation   of,    for   parallel 

operation  of  generators,  64. 
Excitation,  generators,  45. 

rotary  converters,  204. 

synchronous  motors,  147. 
Exciters,   capacities  of,   for  genera- 
tors and  motors,  152. 
Exciters,  methods  of  driving,  50. 
Exciter  panel,  connections,  256,  257. 
Exciting  current,  transformers,  181. 

Farad,  definition,  9. 

Field  excitation  (see  Excitation). 

Flux,  2,  7. 

Frequency,  3,  20. 

Frequency  changers,  224,  228,  231. 

Frequency  changer,  induction  type, 

231. 

Frequency,  choice  of,  329,  333. 
Frequency,  effect  of,  on  cost  of  gen- 
erators, 81. 

effect  of,  on  parallel  operation  of 

generators,  333. 
Frequency,  high,  329. 

induction  motors,  113. 

limit  of,  in  rotary  converters,  214. 

low,  331. 
Fuses,  253. 

Generators,   armature  reaction,   42, 

43- 

armature  inductance,  41. 

armature  windings,  38. 

compound  wound,  connections 
of,  48. 

compounding  of,  45. 

conditions  affecting  cost,  80. 

double  current,  222. 

effect  of  power  factor  on  per- 
formance, 54. 

efficiency,  53. 


INDEX. 


Generators,  electromotive  force,  41. 
elementary  forms,  21. 
excitation  45. 

excitation,  energy  required  for,  50. 
inductor  type,  33. 
losses,  53. 

methods  of  driving,  67. 
monocyclic,  327. 
parallel  running,  58. 
regulation,  52. 

revolving  armature  type,  22. 
revolving  field  type,  24. 
speed,  56. 
speed   regulation    of  engines  for 

parallel  running,  64. 
steam  turbine,  75. 
three-phase,  325. 
three-phase  used  as  single-phase, 

325- 

three-phase  windings,  312. 
two-phase  windings,  299. 

Harmonics,  5. 
Henry,  the,  8. 
Hunting,  definition,  64. 

of  rotary  converters,  221. 

of  synchronous  motors,  155. 

Idle  current  (see  Wattless  current). 

Ilgner  system,  109. 

Impedance,  n. 

Inductance,  7. 

Inductance,  unit  of,  8. 

Induction  factor,  15. 

Induction  motors,  84. 

condensers  for,  117. 

construction  of  primary  and  sec- 
ondary, 91. 

direction  of  rotation,  91. 

efficiency,  115. 

frequency,  113. 


Induction  motors,  initial  voltage,  1 14. 
methods  of  starting,  85. 
power  factor,  115. 
principles  of  operation,  84. 
single-phase,  119. 
speed  regulation,  102,  106. 
starting  torque   and   current,   95, 

99- 

transformer  capacities  for,  116. 
variable  armature  resistance  type, 

85,  89,  101. 
variation  of  starting    torque  with 

change  of  armature  resistance. 

97- 

voltage,  114. 
wiring  for,  115, 
with     short-circuited     armatures, 

90,  101. 

Inductive  loads,  115,  148. 
Inductor  generator,  33. 
Insulators,  line,  289. 
methods  of  testing,  290. 

Lag,  angle  of,  8. 

Lead,  9, 

Lightning    arrester,    cylinder    type, 

285. 

horn  type,  284. 
Lightning  arresters,   installation  of, 

287. 

Lightning  protection,  282. 
Line  (see  also  Transmission  lines), 
constants,    for  power    transmis- 
sion, 344. 
construction,  293. 
protection     of,     from     lightning 

effects,  282. 
Lines  of  force,  7. 
Load,   maximum,  induction  motor, 

99- 
synchronous  motor,  135. 


366 


INDEX. 


Long   distance    power   transmission 
by  three-phase  system,  322. 

by  two-phase  system,  304. 
Losses,  generators,  53. 

induction  motors,  116. 

transformers,  180. 

Magnetic    circuit,    inductor   genera- 
tor* 35i  37- 

revolving  field  generator,  30. 
Magnetic  field,  induction  motor,  84. 
Magnetizing  current,  99,  181. 
Mershon  compensator  (see  Compen- 
sator, line  drop). 
Monocyclic  system,  327. 
Motor  converter,  234. 
Motor  generators,  224. 

advantages  of,  226. 

as  frequency  changers,  228. 

disadvantages  of,  227. 
Multiphase  (see  Polyphase). 

Neutral   point,   in   three-phase   sys- 
tem, 311,  313,  317. 

Ohm's    law,    modifications    of,    in 

alternating-current  circuits,  7. 
Oil  switches,  246. 
Oil    type   transformers,    self -cooled, 

1 60. 

water  cooled,  164. 
Oscillatory    character   of     lightning 

discharge,  282,  288. 
Output,     maximum,     of    induction 

motors,  98,  102. 
of  synchronous  motors,  134. 

Parallel  running  of  generators,  58. 

rotary  converters,  216. 

transformers,  185. 
Periodicity  (see  Frequency). 


Phase     characteristic,     rotary    con- 
verter, 205. 

synchronous  motor,  149. 
Phase,  definition,  6. 
Phase  displacement  (see  Lag). 
Polyphase  circuits,  various  connec- 
tions   of    (see    Two-phase    and 
Three-phase). 

Polyphase    systems    and    combina- 
tions, 298. 

Polyphase  transformers,  157. 
Potential  (see  Voltage). 
Power  factor,  15. 

of  induction   motors,  115. 
rotary  converters,  212. 
synchronous  motors,  152. 
Power     measurement,     three-phase 

system,  318. 
two-phase  system,  307. 
Power  transmission,   long  distance, 

by  three-phase  system,  322. 
long   distance,  by  two-phase   sys- 
tem, 304. 

Pressure  regulators  (see  Regulators),, 
Prime   movers   for   driving   genera- 
tors, 67. 
Punchings,  generator  armature,  39. 

Radiating   surface   of   transformers, 

-158- 

Ratio  of  transformation,  rotary  con- 
verters, 196. 
transformers,  187. 
Reactance,  n. 

effect  of,  on  automatic  compound- 
ing of  rotary  converters,  204, 
208. 

of  transmission  conductors,  344. 
Reaction  (see  Armature  reaction). 
Rectifier,  mercury,  237. 
capacities,  243. 


INDEX. 


367 


Rectifier,  mercury,  limits  of  voltage, 

242. 

Rectifier,  synchronous,  236. 
Regulation,  generators,  52. 

speed,  of  induction  motors,  102. 

speed,     of    synchronous    motors, 

*33»  134- 

transformers,  183. 
Regulator,  feeder,  271. 

efficiency,  279. 

induction  type,  273. 

methods  of  cooling,  279. 

power  factor,  279. 

switch  type,  273. 
Regulator,  Tirrill,  270. 
Resistance,  apparent,  18. 

copper  conductors,  344, 

virtual,  13. 

Resonance,  electrical,  360. 
Reversing  induction  motors,  91. 
Ring  winding,  300,  312. 
Rotary  condenser,  150. 
Rotary  converter,  188. 

armature  reaction,  199,  213. 

automatic  compounding,  208. 

compound  wound  type,  207. 

connections,  189. 

hunting,  221. 

inverted,  188,  221. 

limit  of  frequency,  214. 

methods  of  starting,  217. 

parallel  operation,  216. 

phase  characteristic,  205. 

power  factor,  212. 

ratio  of  AC  to  DC  amperes,  198. 

ratio  of  AC  to  DC  voltage,  192, 196. 

relative   economy   of   material   in 
different  types,  198. 

shunt-wound  type,  204. 

single-phase,  190. 

six-phase,  190. 


Rotary  converter,  three-phase,  190. 
transformer  connections  for,  190. 
two-phase,  190. 

typical     substation     arrangement 
.   for,  268,  269. 

used  for  phase  control,  202,  205. 
variation  of  conversion  ratio,  195, 

196. 

voltage,  regulation  of,  202. 
weights,  216. 
Rotary     transformer     (see     Rotary 

converter). 

Secondary  systems  of  distribution 
(see  Two-phase,  Three-phase, 
etc.). 

Self  induction,  coefficient  of,  8. 
Sine  wave,  2,  3. 
Single-phase   output  of  three-phase 

generators,  325. 
Single-phase     commutator     motors, 

121. 

induction  motors,  119. 
rotary  converter,  190. 
synchronous  motors,  135. 
Six-phase  rotary  converter,  190,  199. 

transformer  connections,  316. 
Skin  effect,  14. 
Slip,  of  induction  motors,  85. 
Speed  control  of  induction  motors, 

102. 

Speed,  effect  of,  on  cost  of  genera- 
tors, 80,  83. 

Speed  regulation  of  engines  for  par- 
allel running,  64. 
Speed  variation  of  induction  motors, 

102. 

Star  connection  of  windings,  300, 312. 
Starting  of  induction  motors,  95. 
of  rotary  converters,  217. 
of  synchronous  motors,  136. 


368 


INDEX. 


Starting  current  induction   motors, 

95,  99- 

synchronous  motors,  138. 
Static     transformers     (see     Trans- 
formers). 

Station  equipment,  244. 
Switchboards,  244,  253. 
Switchboard  equipment,  location  of, 

264. 
Switchboards,     typical     connection 

diagrams  of,  256. 
Switches,  245. 
air  break,  245. 
oil,  246. 

Synchronism  indicator,  63. 
Synchronizing,  methods  of,  60,  64. 
Synchronous  motor,  133. 
Synchronous  motors,  advantages  of, 

133- 

amortisseur  winding  for,  146. 

disadvantages  of,  133. 

excitation,  147. 

hunting,  155. 

methods  of  starting,  136. 

phase  characteristic,  149. 

power  factor,  152. 

speed,  134. 

torque  and  output,  134. 

used  as  condenser,  150. 

voltage,  135. 
Three-phase  curves  of  E.M.F.,  311. 

four- wire  system,  314. 

long    distance     transmission    cir- 
cuits, 321. 

motor  connections,  316. 

three-wire  system,  312. 

transformer  connect'ons,  312. 
Three-phase  circuits,  for  power  distri- 
bution, 321. 

for  lighting  distribution,  324. 

for  railway  distribution,  262. 


Three-phase  system,  311. 
Three-phase    system,    measurement 

of  power  in,  318. 
Torque  diagram  of  induction  motors, 

97. 

Transformers,  157. 
Transformers  — 

air  blast,  173. 

air  blast,  amount  of  air  required 
for,  176. 

air  blast,  operation  of,  178. 

efficiency,  180. 

exciting  current,  181. 

high  voltage  types  for  testing  use, 
169. 

losses,  1 80. 

oil  type,  quality  of  oil  required  for, 
170. 

parallel  operation,  185. 

reasons    for    artificial    cooling    in 
large  sizes,  158. 

regulation,  183. 

self -cooled  oil  type,  160. 

structure     of     magnetic     circuit, 
179. 

variable  ratio,  187. 

water-cooled  oil  type,  164. 
Transformer  connections  — 

six-phase,  316. 

three-phase,  312. 

two-phase,  301. 

two-phase  to  three  phase,  302. 
Transmission   lines,    calculation   of, 
34i. 

capacity  of,  344. 

constants  for,  344. 

charging  current  in,  344. 

inductance  of,  344. 

resistance  of,  344.   ' 

voltage  drop  in,  341,  356. 
Two-phase,    four-wire   system,  304 


INDEX. 


Two-phase  generator  armature  con- 
nections, 2QQ. 

interlinked  windings,  299. 

separate  windings,  299. 

three-wire  system,  307. 

to  three-phase,  302. 

transformer  connections,  301. 

unbalancing,  308. 
Two-phase  system,  298. 

Virtual  resistance,  13. 

Voltage     (see     also    Electromotive 

Force). 
Voltage,    automatic    regulation    of, 

270. 
drop   in   transmission   lines,    341, 

356. 
effects  of,  on  output  of  induction 

motor,  114. 

of  induction  motor,  1 14. 
of  synchronous  motor,  135. 
Voltage  relation  of  line  to  induced 
E.M.F.    in   three-phase    gener- 
ator, 43. 


Voltage,  rise  of,  on  opening  a  circuit, 

251. 

variation  of,  in  transmission  lines 
due  to  phase  displacement,  17,20. 

Water  wheels,  as  prime  movers,  60, 
67,  70. 

Wattless  current,  16. 

Wattless  current  of  induction  motor, 

117. 
of  transformer,  181. 

Wattmeter     (see     Power     measure- 
ment). 

Watts,  apparent,  14. 

Wave  form,  6. 

Weights,  of  copper  required  for  vari- 
ous systems,  335, 

Windings  (see  Three-phase  and  Two- 
phase). 

Wiring  formulas,  347. 
application  of,  350. 

Y  connection  in  three-phase  system, 
312. 


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tion of  Incandescent  Stations,  by  0.  J.  Field  ;  a  Description  of  the  Edison  Elec- 
trolyte Meter,  by  A.  E.  Kennelly ;  and  a  Paper  on  the  Maximum  Efficiency  of 
Incandescent  Lamps,  by  T.  W.  Howells.  Fifth  edition.  Illustrated.  16mo, 
cloth.  (No.  57  Van  Nostrand's  Series. ), $0.50 

JEHL,  FRANCIS.  Member  A.  I.  E.  E.  The  manufacture  of  Carbons  for  Elec- 
tric Lighting  and  other  purposes.  A  Practical  Hand-book,  giving  a  complete 
description  of  the  art  of  making  carbons,  electros,  etc.  The  various  gas  gene- 
rators and  furnaces  used  in  carbonising,  with  a  plan  for  a  model  factory.  Illus- 
trated with  numerous  diagrams,  tables,  and  folding  plates.  8vo,  cloth.  Illus- 
trated,   $4.00 

KAPP,  GISBERT,  C.  E.  Electric  Transmission  of  Energy  and  its  Transforma- 
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revised.  12mo,  cloth $3.50 

Alternate-Current  Machinery.  190  pages.  Illustrated.  (No.  96  Van  Nos- 
trand's Science  Series.), $0.50 

Dynamos,  Alternators,  and  Transformers.    Illustrated.    8vo,  cloth,  .  .  .  $4.00 

KEMPE,  H.  R.  The  Electrical  Engineer's  Pocket-book ;  Modern  Rules,  Formulae, 
Tables,  and  Data.  32mo,  leather, $1.75 

KENNELLY,  A.  E.  Theoretical  Elements  of  Electro-dynamic  Machinery.  Vol. 
I.  Illustrated.  8vo,  cloth, $1.50 

u  1 1  4. oi  It.  M.  H.,  and  SWAN,  H.,  and  BIGGS,  C.  H.  W.  Electrical  Dis- 
tribution :  Its  Theory  and  Practice.  Illustrated.  8vo,  cloth,  .......  $4.00 

LEVY,  C.  L.  Electric  Light  Primer.  A  simple  and  comprehensive  digest  of  all 
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tric lights,  with  precautions  for  safety.  For  the  use  of  persons  whose  duty  it  is 
to  look  after  the  plant.  8vo,  paper, $0.50 

LOCKWOOD,  T.  D.  Electricity,  Magnetism,  and  Electro-telegraphy.  A  Prac- 
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Operators,  and  Inspectors.  Revised  edition.  8vo,  cloth.  Profusely  Illus- 
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LODGE,  PROF.  OLIVER  J.  Signalling  Across  Space  Without  Wires :  being 
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cloth, $2.00 

LORING,  A.  E.  A  Hand-book  of  the  Electro-magnetic  Telegraph.  Fourth  edi- 
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cloth.  (No.  64  Van  Nostrand's  Science  Series.), $0.50 

MORROW,  J.  T.,  and  REID,  T.  Arithmetic  of  Magnetism  and  Electricity. 
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M  i*  1 1  I :  ic .  FRANCIS  E.,  A.  M.  Theory  of  Magnetic  Measurements,  with  an 
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OHM,  Dr.  G.  S.  The  Galvanic  Circuit  Investigated  Mathematically.  Berlin, 
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Thos.  D.  Lockwood.  16mo,  cloth.  (No.  102  Van  Nostrand's  Science  Series.),  .  $0.50 

OUDIN,  MAURICE  A.  Standard  Polyphase  Apparatus  and  Systems,  con- 
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PARSHALL,  H.  F.,  and  HOBART,  H.  M.  Armature  Windings  of  Elec- 
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POPE,  F.  L.  Modern  Practice  of  the  Electric  Telegraph.  A  Hand-book  for 
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POOliE,  J.    The  Practical  Telephone  Hand-book.    Illustrated.  8vo,  cloth,    $1.50 

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SALOMONS,  Sir  DAVID,  M.  A-  Electric-light  Installations.  Vol.  I.  Man- 
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4 


SALOMONS,  Sir  DAVID,  M.  A.     fol.  II. :  Apparatus 82.29 

Vol.  III.:  Application 81.50 

8CHELLEN,  Dr.  H.  Magneto-electric  and  Dynamo-electric  Machines;  Their 
Construction  and  Practical  Application  to  Electric  Lighting  and  the  Trans- 
mission of  Power.  Translated  from  the  third  German  edition  by  N.  S. 
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lating to  American  Machines,  by  N.  S.  Keith.  Vol.  I,  with  353  Illustrations. 
Second  edition $5.00 

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SLOANE,  Prof.  T.  O'CONOR.  Standard  Electrical  Dictionary.  300  Illus- 
trations. 8vo,  cloth $3.00 

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tion on  the  Applications  of  Electricity  to  Mining  Work.  Second  edition.  8vo, 
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SWOOPE,  C.  W.  Lessons  in  Practical  Electricity :  Principles,  Experiments 
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THOMPSON,  Prof.  S.  P.  Dynamo-electric  Machinery.  With  an  Introduc- 
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TUNZELMANN,  G.  W.  de.  Electricity  in  Modern  Life.  Illustrated.  12mo, 
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of  Voltaic  Batteries,  Magneto  and  Dynamo-Electric  Machines,  Thermopiles, 
and  of  the  Materials  and  Processes  used  in  every  Department  of  the  Art,  and 
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to  Wind  for  any  Output.  Illustrated.  16ino,  cloth,  (No.  98  Van  Nostrand's 
Science  Series.) $0.50 

WALKER,  SYDNEY  F.  Electricity  in  our  Homes  and  Workshops.  A  Prac- 
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WEBB,  H.  L.  A  Practical  Guide  to  the  Testing  of  Insulated  Wires  and  Cables. 
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WILKINSON,  H.  D.  Submarine  Cable-Laying,  Repairing,  and  Testing.  8vo, 
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WORM  E LL.  R.  Electricity  in  the  Service  of  Man.  A  Popular  and  Practical 
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YOUNG,  J.  ELTON.  Electrical  Testing  for  Telegraph  Engineers.  With  Ap- 
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